resolve_ref(): expand documentation

Record information about resolve_ref(), hard-won via reverse
engineering, in a comment for future spelunkers.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -831,7 +831,39 @@ extern int get_sha1_hex(const char *hex, unsigned char *sha1);
 
 extern char *sha1_to_hex(const unsigned char *sha1);	/* static buffer result! */
 extern int read_ref(const char *filename, unsigned char *sha1);
-extern const char *resolve_ref(const char *path, unsigned char *sha1, int, int *);
+
+/*
+ * Resolve a reference, recursively following symbolic refererences.
+ *
+ * Store the referred-to object's name in sha1 and return the name of
+ * the non-symbolic reference that ultimately pointed at it.  The
+ * return value, if not NULL, is a pointer into either a static buffer
+ * or the input ref.
+ *
+ * If the reference cannot be resolved to an object, the behavior
+ * depends on the "reading" argument:
+ *
+ * - If reading is set, return NULL.
+ *
+ * - If reading is not set, clear sha1 and return the name of the last
+ *   reference name in the chain, which will either be a non-symbolic
+ *   reference or an undefined reference.  If this is a prelude to
+ *   "writing" to the ref, the return value is the name of the ref
+ *   that will actually be created or changed.
+ *
+ * If flag is non-NULL, set the value that it points to the
+ * combination of REF_ISPACKED (if the reference was found among the
+ * packed references) and REF_ISSYMREF (if the initial reference was a
+ * symbolic reference).
+ *
+ * If ref is not a properly-formatted, normalized reference, return
+ * NULL.  If more than MAXDEPTH recursive symbolic lookups are needed,
+ * give up and return NULL.
+ *
+ * errno is sometimes set on errors, but not always.
+ */
+extern const char *resolve_ref(const char *ref, unsigned char *sha1, int reading, int *flag);
+
 extern int dwim_ref(const char *str, int len, unsigned char *sha1, char **ref);
 extern int dwim_log(const char *str, int len, unsigned char *sha1, char **ref);
 extern int interpret_branch_name(const char *str, struct strbuf *);