canonicalize: improve documentation

canonicalize: improve documentation

[Bruno Haible]

Once the failure return convention is known for xgetcwd(), it follows also
for canonicalize_filename_mode().


2020-06-26  Bruno Haible  <bruno@clisp.org>

	canonicalize: Improve documentation.
	* lib/canonicalize.h (canonicalize_filename_mode): Document the failure
	return convention.

diff --git a/lib/canonicalize.h b/lib/canonicalize.h
index 950d117..1815297 100644
--- a/lib/canonicalize.h
+++ b/lib/canonicalize.h
@@ -46,7 +46,8 @@ typedef enum canonicalize_mode_t canonicalize_mode_t;
    does not contain any `.', `..' components nor any repeated file name
    separators ('/') or, depending on other CAN_MODE flags, symlinks.
    Whether components must exist or not depends on canonicalize mode.
-   The result is malloc'd.  */
+   The result is malloc'd.
+   Upon failure, return NULL with errno set.  */
 char *canonicalize_filename_mode (const char *, canonicalize_mode_t);
 
 #ifdef __cplusplus