Document compat_getdelim_.

src/lib/fs/files.h

@@ -104,6 +104,9 @@ char *read_file_to_str_until_eof(int fd, size_t max_bytes_to_read,
   ATTR_MALLOC;
 
 #if !defined(HAVE_GETDELIM) || defined(TOR_UNIT_TESTS)
+/** Internal back-end function to implement getdelim(): only exists when
+ * Tor is built for unit tests, or when Tor is built on an operating system
+ * without its own getdelim(). */
 ssize_t compat_getdelim_(char **lineptr, size_t *n, int delim, FILE *stream);
 #endif
 
@@ -112,6 +115,8 @@ ssize_t compat_getdelim_(char **lineptr, size_t *n, int delim, FILE *stream);
  * Cross-platform wrapper for getdelim(): behaves as the POSIX-standard
  * getdelim() function.
  *
+ * See `getdelim(3)` for more information.
+ *
  * Note that this function will use the libc memory allocator -- so any memory
  * passed to this function must come from raw_malloc(), and must be freed by
  * raw_free() -- don't use tor_malloc() and tor_free() with this.