Address review comment: clarify realloc_and_append str_to_add=NULL behavior

When _no_user_buf is false and str_to_add is NULL, the function is a
no-op returning buf unchanged; "ensure space" only applies in
library-managed mode.

Author: brtnfld

hl/src/H5LT.c

@@ -1919,7 +1919,9 @@ H5LTtext_to_dtype(const char *text, H5LT_lang_t lang_type)
  *                 - false: User-provided buffer (fixed size, no realloc)
  *   len:         Pointer to current buffer size (updated if reallocated)
  *   buf:         Buffer to append to (must not be NULL)
- *   str_to_add:  String to append (NULL = no append, just ensure space)
+ *   str_to_add:  String to append. If NULL:
+ *                  - Library-managed mode: reallocates to ensure extra space, no append
+ *                  - User-provided mode: no-op, returns buf unchanged (space cannot be ensured)
  *
  * Preconditions:
  *   - buf must be a valid, allocated buffer (never NULL)