Plug-ins: plugin.install's params text and docs/mcp.md state when a grant is kept

A refused install keeps the grant, a failed write of the module to the
plug-in store leaves the grant in the store, and a stored module that is
disabled or not read in Safe Mode keeps the grant for its id. This
matches docs/plugins.md.

crates/engine/src/cmd/plugin.rs

@@ -942,7 +942,7 @@ pub fn specs() -> Vec<CommandSpec> {
             "Install Plug-in",
             [],
             None,
-            "{path: a .wasm file (desktop) | dataBase64: the module (+ name?: shown as its source), replace?: bool (true once the user agreed: a module with other bytes than the plug-in installed with its id, or with none installed, one whose id has settings or a cache in the plug-in store, installs only with it, takes that plug-in's place and gets its settings and cache; false: one whose id is installed is refused; left out, the same module installs again), name?: file name (with dataBase64), allowHeavy?: bool (true once the user agreed to a heavy budget: a filter whose manifest asks for one, budget \"heavy\" in plugin.inspect, installs only with it), grant?: [permission] (the permissions the user granted: a plug-in whose manifest lists permissions, needsApproval in plugin.inspect, installs only when grant holds every one of them; the grant is recorded with the module's hash and covers only that module, so a module with other bytes needs a new grant even when it lists the same permissions; installing a module that lists no permissions forgets the grant held for its id; with a plug-in store attached, only the grants kept there count), hash?: string (the module's SHA-256 as plugin.inspect gave it: a module with other bytes is refused)} validate and install a WebAssembly plug-in for this session → its plugin.list entry",
+            "{path: a .wasm file (desktop) | dataBase64: the module (+ name?: shown as its source), replace?: bool (true once the user agreed: a module with other bytes than the plug-in installed with its id, or with none installed, one whose id has settings or a cache in the plug-in store, installs only with it, takes that plug-in's place and gets its settings and cache; false: one whose id is installed is refused; left out, the same module installs again), name?: file name (with dataBase64), allowHeavy?: bool (true once the user agreed to a heavy budget: a filter whose manifest asks for one, budget \"heavy\" in plugin.inspect, installs only with it), grant?: [permission] (the permissions the user granted: a plug-in whose manifest lists permissions, needsApproval in plugin.inspect, installs only when grant holds every one of them; the grant is recorded with the module's hash and covers only that module, so a module with other bytes needs a new grant even when it lists the same permissions; installing a module that lists no permissions forgets the grant held for its id; a refused install keeps the grant, a failed write of the module to the plug-in store leaves the grant in the store, and a stored module that is disabled or not read in Safe Mode keeps the grant for its id; with a plug-in store attached, only the grants kept there count), hash?: string (the module's SHA-256 as plugin.inspect gave it: a module with other bytes is refused)} validate and install a WebAssembly plug-in for this session → its plugin.list entry",
             always,
             install
         ),

docs/mcp.md

@@ -6710,7 +6710,9 @@ an error naming the permissions needed. The grant is kept with the module's hash
 same bytes install again without `grant`, and a module with other bytes has `needsApproval: true` and needs a new
 `grant` even when it lists the same permissions. `plugin.install` does not check that a person approved `grant`: it
 trusts the client that sends it. `plugin.remove` forgets the grant, and so does installing a module that lists no
-permissions under that id: the earlier bytes then need a new `grant`. When `plugin.remove` can't remove the grant from
+permissions under that id: the earlier bytes then need a new `grant`. A refused install keeps the grant. A failed write
+of the module to the plug-in store leaves the grant in the store. A stored module that is disabled, or not read in Safe
+Mode, keeps the grant for its id. When `plugin.remove` can't remove the grant from
 the plug-in store, it returns an error and the plug-in stays installed. A
 filter without `doc.write` or `state` that returns output needing them gets an error and the document is unchanged.
 A plug-in of API level 2 (`api: 2` in its entry) that lists `progress` may report its progress, see Cancel and write