docs(lsp): document rpc Client request/notify as fields #40573

Problem:
`request()` and `notify()` are methods of the object returned by
`vim.lsp.rpc.start()`/`connect()`, but were rendered with module-level
helptags (`vim.lsp.rpc.request()`, `vim.lsp.rpc.notify()`) (erroneously
implying module functions that do not exist).

Solution:
Mark the wrappers `@private` and describe them on `vim.lsp.rpc.Client` instead.

runtime/lua/vim/lsp/client.lua

@@ -72,7 +72,7 @@ end
 --- end
 --- ```
 ---
---- - See |vim.lsp.rpc.request()| |vim.lsp.rpc.notify()|
+--- - See |vim.lsp.rpc.Client|.
 --- - For TCP there is a builtin RPC client factory: |vim.lsp.rpc.connect()|
 --- @field cmd string[]|fun(dispatchers: vim.lsp.rpc.Dispatchers, config: vim.lsp.ClientConfig): vim.lsp.rpc.Client
 ---