doc: Document actual LspStopServer behavior (#1490)

Actual behavior was slightly different from the documented one.

Co-authored-by: mattn <[email protected]>

doc/vim-lsp.txt

@@ -1909,9 +1909,17 @@ Prints the status of all registered servers. Use `:verbose LspStatus` to
 additionally show each server's workspace_config.
 See also |vim-lsp-healthcheck|.
 
-LspStopServer                                                *:LspStopServer*
+LspStopServer [name]                                         *:LspStopServer*
 
-Stop all active servers.
+If 'name' is not specified, then all active servers that handle files matching
+the current buffer type are stopped.  This is often what you want.  For
+example, if you have multiple files of different types open, `LspStopServer`
+will only stop the server for the current buffer.
+
+When 'name' is provided, it acts as an additional restriction, only stopping
+server that handles the current buffer type, if it also matches the specifie
+name.  'name' value is compred to the 'name' property in the
+|lsp#register_server()| call.
 
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 Autocommands                                          *vim-lsp-autocommands*