date API: add basic API docs

Add basic API doc comments to date.h, and while doing so move the the
parse_date_format() function adjacent to show_date(). This way all the
"struct date_mode" functions are grouped together. Documenting the
rest is one of our #leftoverbits.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

date.h

@@ -1,6 +1,12 @@
 #ifndef DATE_H
 #define DATE_H
 
+/**
+ * The date mode type. This has DATE_NORMAL at an explicit "= 0" to
+ * accommodate a memset([...], 0, [...]) initialization when "struct
+ * date_mode" is used as an embedded struct member, as in the case of
+ * e.g. "struct pretty_print_context" and "struct rev_info".
+ */
 enum date_mode_type {
 	DATE_NORMAL = 0,
 	DATE_HUMAN,
@@ -24,7 +30,7 @@ struct date_mode {
 	.type = DATE_NORMAL, \
 }
 
-/*
+/**
  * Convenience helper for passing a constant type, like:
  *
  *   show_date(t, tz, DATE_MODE(NORMAL));
@@ -32,7 +38,22 @@ struct date_mode {
 #define DATE_MODE(t) date_mode_from_type(DATE_##t)
 struct date_mode *date_mode_from_type(enum date_mode_type type);
 
+/**
+ * Format <'time', 'timezone'> into static memory according to 'mode'
+ * and return it. The mode is an initialized "struct date_mode"
+ * (usually from the DATE_MODE() macro).
+ */
 const char *show_date(timestamp_t time, int timezone, const struct date_mode *mode);
+
+/**
+ * Parse a date format for later use with show_date().
+ *
+ * When the "date_mode_type" is DATE_STRFTIME the "strftime_fmt"
+ * member of "struct date_mode" will be a malloc()'d format string to
+ * be used with strbuf_addftime().
+ */
+void parse_date_format(const char *format, struct date_mode *mode);
+
 void show_date_relative(timestamp_t time, struct strbuf *timebuf);
 int parse_date(const char *date, struct strbuf *out);
 int parse_date_basic(const char *date, timestamp_t *timestamp, int *offset);
@@ -41,7 +62,6 @@ void datestamp(struct strbuf *out);
 #define approxidate(s) approxidate_careful((s), NULL)
 timestamp_t approxidate_careful(const char *, int *);
 timestamp_t approxidate_relative(const char *date);
-void parse_date_format(const char *format, struct date_mode *mode);
 int date_overflows(timestamp_t date);
 time_t tm_to_time_t(const struct tm *tm);
 #endif