common & yang DOC minor updates
diff --git a/src/common.h b/src/common.h
index a15fb72..64ea5a9 100644
--- a/src/common.h
+++ b/src/common.h
@@ -145,10 +145,10 @@
 };
 
 enum yang_arg {
-    Y_IDENTIF_ARG,
-    Y_PREF_IDENTIF_ARG,
-    Y_STR_ARG,
-    Y_MAYBE_STR_ARG
+    Y_IDENTIF_ARG,        /**< YANG "identifier-arg-str" rule */
+    Y_PREF_IDENTIF_ARG,   /**< YANG "identifier-ref-arg-str" rule */
+    Y_STR_ARG,            /**< YANG "string" rule */
+    Y_MAYBE_STR_ARG       /**< optional YANG "string" rule */
 };
 
 enum yang_keyword {
diff --git a/src/parser_yang.c b/src/parser_yang.c
index 7f88bc6..9ffe234 100644
--- a/src/parser_yang.c
+++ b/src/parser_yang.c
@@ -450,7 +450,7 @@
  * @param[in] ctx libyang context for logging.
  * @param[in,out] data Data to read from, always moved to currently handled character.
  * @param[in] arg Type of YANG keyword argument expected.
- * @param[out] word_p Pointer to the read string.
+ * @param[out] word_p Pointer to the read string. Can be NULL if \p arg is #Y_MAYBE_STR_ARG.
  * @param[out] word_b Pointer to a dynamically-allocated buffer holding the read string. If not needed,
  * set to NULL. Otherwise equal to \p word_p.
  * @param[out] word_len Length of the read string.