gchecksum: Add missing (transfer) and (nullable) return annotations

This commit only looks at the `Returns:` lines in the documentation, and
has examined all of them in the file. Function arguments have not been
checked.
Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

Helps: #2227

glib/gchecksum.c

@@ -1461,7 +1461,7 @@ g_checksum_type_get_length (GChecksumType checksum_type)
  * will be closed and it won't be possible to call g_checksum_update()
  * on it anymore.
  *
- * Returns: (transfer full): the newly created #GChecksum, or %NULL.
+ * Returns: (transfer full) (nullable): the newly created #GChecksum, or %NULL.
  *   Use g_checksum_free() to free the memory allocated by it.
  *
  * Since: 2.16
@@ -1529,8 +1529,8 @@ g_checksum_reset (GChecksum *checksum)
  * g_checksum_get_string() or g_checksum_get_digest(), the copied
  * checksum will be closed as well.
  *
- * Returns: the copy of the passed #GChecksum. Use g_checksum_free()
- *   when finished using it.
+ * Returns: (transfer full): the copy of the passed #GChecksum. Use
+ *   g_checksum_free() when finished using it.
  *
  * Since: 2.16
  */
@@ -1775,8 +1775,10 @@ g_checksum_get_digest (GChecksum  *checksum,
  *
  * The hexadecimal string returned will be in lower case.
  *
- * Returns: the digest of the binary data as a string in hexadecimal.
- *   The returned string should be freed with g_free() when done using it.
+ * Returns: (transfer full) (nullable): the digest of the binary data as a
+ *   string in hexadecimal, or %NULL if g_checksum_new() fails for
+ *   @checksum_type. The returned string should be freed with g_free() when
+ *   done using it.
  *
  * Since: 2.16
  */
@@ -1812,7 +1814,8 @@ g_compute_checksum_for_data (GChecksumType  checksum_type,
  *
  * The hexadecimal string returned will be in lower case.
  *
- * Returns: the checksum as a hexadecimal string. The returned string
+ * Returns: (transfer full) (nullable): the checksum as a hexadecimal string,
+ *   or %NULL if g_checksum_new() fails for @checksum_type. The returned string
  *   should be freed with g_free() when done using it.
  *
  * Since: 2.16
@@ -1842,8 +1845,10 @@ g_compute_checksum_for_string (GChecksumType  checksum_type,
  *
  * The hexadecimal string returned will be in lower case.
  *
- * Returns: the digest of the binary data as a string in hexadecimal.
- *   The returned string should be freed with g_free() when done using it.
+ * Returns: (transfer full) (nullable): the digest of the binary data as a
+ *   string in hexadecimal, or %NULL if g_checksum_new() fails for
+ *   @checksum_type. The returned string should be freed with g_free() when
+ *   done using it.
  *
  * Since: 2.34
  */