strbuf_branchname: add docstring

This function and its companion, strbuf_check_branch_ref(),
did not have their purpose or semantics explained. Let's do
so.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

strbuf.h

@@ -562,7 +562,22 @@ static inline void strbuf_complete_line(struct strbuf *sb)
 	strbuf_complete(sb, '\n');
 }
 
+/*
+ * Copy "name" to "sb", expanding any special @-marks as handled by
+ * interpret_branch_name(). The result is a non-qualified branch name
+ * (so "foo" or "origin/master" instead of "refs/heads/foo" or
+ * "refs/remotes/origin/master").
+ *
+ * Note that the resulting name may not be a syntactically valid refname.
+ */
 extern void strbuf_branchname(struct strbuf *sb, const char *name);
+
+/*
+ * Like strbuf_branchname() above, but confirm that the result is
+ * syntactically valid to be used as a local branch name in refs/heads/.
+ *
+ * The return value is "0" if the result is valid, and "-1" otherwise.
+ */
 extern int strbuf_check_branch_ref(struct strbuf *sb, const char *name);
 
 extern void strbuf_addstr_urlencode(struct strbuf *, const char *,