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`.

src/nvim/lua/executor.c

@@ -798,9 +798,9 @@ static void typval_exec_lua(const char *lcmd, size_t lcmd_len, const char *name,
   }
 }
 
-/// Execute lua string
+/// Execute Lua string
 ///
-/// Used for nvim_execute_lua().
+/// Used for nvim_exec_lua().
 ///
 /// @param[in]  str  String to execute.
 /// @param[in]  args array of ... args