strbuf doc: reuse after strbuf_release is fine

strbuf_release leaves the strbuf in a valid, initialized state, so
there is no need to call strbuf_init after it.

Moreover, this is not likely to change in the future: strbuf_release
leaving the strbuf in a valid state has been easy to maintain and has
been very helpful for Git's robustness and simplicity (e.g.,
preventing use-after-free vulnerabilities).

Document the semantics so the next generation of Git developers can
become familiar with them without reading the implementation.  It is
still not advisable to call strbuf_release too often because it is
wasteful, so add a note pointing to strbuf_reset for that.

The same semantics apply to strbuf_detach.  Add a similar note to its
docstring to make that clear.

Improved-by: Jeff King <peff@peff.net>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

strbuf.h

@@ -82,8 +82,12 @@ extern char strbuf_slopbuf[];
 extern void strbuf_init(struct strbuf *, size_t);
 
 /**
- * Release a string buffer and the memory it used. You should not use the
- * string buffer after using this function, unless you initialize it again.
+ * Release a string buffer and the memory it used. After this call, the
+ * strbuf points to an empty string that does not need to be free()ed, as
+ * if it had been set to `STRBUF_INIT` and never modified.
+ *
+ * To clear a strbuf in preparation for further use without the overhead
+ * of free()ing and malloc()ing again, use strbuf_reset() instead.
  */
 extern void strbuf_release(struct strbuf *);
 
@@ -91,6 +95,9 @@ extern void strbuf_release(struct strbuf *);
  * Detach the string from the strbuf and returns it; you now own the
  * storage the string occupies and it is your responsibility from then on
  * to release it with `free(3)` when you are done with it.
+ *
+ * The strbuf that previously held the string is reset to `STRBUF_INIT` so
+ * it can be reused after calling this function.
  */
 extern char *strbuf_detach(struct strbuf *, size_t *);