API: rename nvim_execute_lua => nvim_exec_lua

- We already find ourselves renaming nvim_execute_lua in tests and
  scripts, which suggests "exec" is the verb we actually want.
- Add "exec" verb to `:help dev-api`.

runtime/doc/api.txt

@@ -483,7 +483,8 @@ nvim_exec({src}, {output})                                       *nvim_exec()*
                 Executes Vimscript (multiline block of Ex-commands), like
                 anonymous |:source|.
 
-                Optionally returns (non-error, non-shell |:!|) output.
+                Unlike |nvim_command()| this function supports heredocs,
+                script-scope (s:), etc.
 
                 On execution error: fails with VimL error, does not update
                 v:errmsg.
@@ -493,6 +494,10 @@ nvim_exec({src}, {output})                                       *nvim_exec()*
                     {output}  Capture and return all (non-error, non-shell
                               |:!|) output
 
+                Return: ~
+                    Output (non-error, non-shell |:!|) if `output` is true,
+                    else empty string.
+
                 See also: ~
                     |execute()|
                     |nvim_command()|
@@ -643,7 +648,7 @@ nvim_eval({expr})                                                *nvim_eval()*
                 Return: ~
                     Evaluation result or expanded object
 
-nvim_execute_lua({code}, {args})                          *nvim_execute_lua()*
+nvim_exec_lua({code}, {args})                                *nvim_exec_lua()*
                 Execute Lua code. Parameters (if any) are available as `...`
                 inside the chunk. The chunk can return a value.
 

runtime/doc/deprecated.txt

@@ -15,6 +15,7 @@ updated.
 API ~
 *nvim_buf_clear_highlight()*	Use |nvim_buf_clear_namespace()| instead.
 *nvim_command_output()*		Use |nvim_exec()| instead.
+*nvim_execute_lua()*		Use |nvim_exec_lua()| instead.
 
 Commands ~
 *:rv*

runtime/doc/develop.txt

@@ -236,10 +236,11 @@ with a {thing} that groups functions under a common concept).
 
 Use existing common {action} names if possible:
     add   Append to, or insert into, a collection
-    get   Get a thing (or group of things by query)
-    set   Set a thing (or group of things)
     del   Delete a thing (or group of things)
+    exec  Execute code
+    get   Get a thing (or group of things by query)
     list  Get all things
+    set   Set a thing (or group of things)
 
 Use consistent names for {thing} in all API functions. E.g. a buffer is called
 "buf" everywhere, not "buffer" in some places and "buf" in others.