docs: fix typos (#19798)

Co-authored-by: adrian5 <adrian5@users.noreply.github.com>
Co-authored-by: Eisuke Kawashima <e-kwsm@users.noreply.github.com>

src/nvim/buffer_defs.h

@@ -366,8 +366,8 @@ struct mapblock {
   char m_nowait;                // <nowait> used
   char m_expr;                  // <expr> used, m_str is an expression
   sctx_T m_script_ctx;          // SCTX where map was defined
-  char *m_desc;                 // description of keymap
-  bool m_replace_keycodes;      // replace termcodes in lua function
+  char *m_desc;                 // description of mapping
+  bool m_replace_keycodes;      // replace keycodes in result of expression
 };
 
 /// Used for highlighting in the status line.