cred: Update some comments after 'cr_gid' is no more cr_groups[0]

While here, fix style of some already-updated comments.

Fixes:          be1f7435ef ("kern: start tracking cr_gid outside of cr_groups[]")
MFC after:      5 days
MFC to:         stable/15
Sponsored by:   The FreeBSD Foundation
Differential Revision:  https://reviews.freebsd.org/D52266

sys/kern/kern_prot.c

@@ -1803,12 +1803,6 @@ groupmember(gid_t gid, const struct ucred *cred)
 bool
 realgroupmember(gid_t gid, const struct ucred *cred)
 {
-	/*
-	 * Although the equality test on 'cr_rgid' below doesn't access
-	 * 'cr_groups', we check for the latter's length here as we assume that,
-	 * if 'cr_ngroups' is 0, the passed 'struct ucred' is invalid, and
-	 * 'cr_rgid' may not have been filled.
-	 */
 	groups_check_positive_len(cred->cr_ngroups);
 
 	if (gid == cred->cr_rgid)
@@ -2921,8 +2915,8 @@ crextend(struct ucred *cr, int n)
  * Normalizes a set of groups to be applied to a 'struct ucred'.
  *
  * Normalization ensures that the supplementary groups are sorted in ascending
- * order and do not contain duplicates.  This allows group_is_supplementary
- * to do a binary search.
+ * order and do not contain duplicates.  This allows group_is_supplementary() to
+ * do a binary search.
  */
 static void
 groups_normalize(int *ngrp, gid_t *groups)
@@ -2985,9 +2979,9 @@ crsetgroups_internal(struct ucred *cr, int ngrp, const gid_t *groups)
  * Copy groups in to a credential after expanding it if required.
  *
  * May sleep in order to allocate memory (except if, e.g., crextend() was called
- * before with 'ngrp' or greater).  Truncates the list to ngroups_max if
+ * before with 'ngrp' or greater).  Truncates the list to 'ngroups_max' if
  * it is too large.  Array 'groups' doesn't need to be sorted.  'ngrp' must be
- * strictly positive.
+ * positive.
  */
 void
 crsetgroups(struct ucred *cr, int ngrp, const gid_t *groups)
@@ -3018,8 +3012,8 @@ crsetgroups(struct ucred *cr, int ngrp, const gid_t *groups)
  * Same as crsetgroups() but sets the effective GID as well.
  *
  * This function ensures that an effective GID is always present in credentials.
- * An empty array will only set the effective GID to the default_egid, while a
- * non-empty array will peel off groups[0] to set as the effective GID and use
+ * An empty array will only set the effective GID to 'default_egid', while
+ * a non-empty array will peel off groups[0] to set as the effective GID and use
  * the remainder, if any, as supplementary groups.
  */
 void