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.

src/nvim/mapping.c

@@ -293,7 +293,7 @@ static bool set_maparg_lhs_rhs(const char *const orig_lhs, const size_t orig_lhs
       replaced = replace_termcodes(orig_rhs, orig_rhs_len, &rhs_buf, REPTERM_DO_LT, NULL,
                                    cpo_flags);
       mapargs->rhs_len = STRLEN(replaced);
-      // XXX: replace_termcodes may produce an empty string even if orig_rhs is non-empty
+      // NB: replace_termcodes may produce an empty string even if orig_rhs is non-empty
       // (e.g. a single ^V, see :h map-empty-rhs)
       mapargs->rhs_is_noop = orig_rhs_len != 0 && mapargs->rhs_len == 0;
       mapargs->rhs = (char_u *)replaced;