patch 8.2.0775: not easy to call a Vim function from Lua

Problem:    Not easy to call a Vim function from Lua.
Solution:   Add vim.call() and vim.fn(). (Prabir Shrestha, closes #6063)

runtime/doc/if_lua.txt

@@ -199,6 +199,15 @@ Vim evaluation and command execution, and others.
 				returns it. Note that the buffer is not set as
 				current.
 
+	vim.call({name} [,{args}])
+				Proxy to call Vim function named {name} with 
+				arguments {args}.  Example: >
+					:lua print(vim.call('has', 'timers'))
+<
+	vim.fn			Proxy to call Vim functions. Proxy methods are
+				created on demand.  Example: >
+					:lua print(vim.fn.has('timers'))
+<
 
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 3. List userdata					*lua-list*