scripts: kernel-doc: add warning for comment not following kernel-doc syntax

Currently, kernel-doc start parsing the comment as a kernel-doc comment if
it starts with '/**', but does not take into account if the content inside
the comment too, adheres with the expected format.
This results in unexpected and unclear warnings for the user.

E.g., running scripts/kernel-doc -none mm/memcontrol.c emits:
"mm/memcontrol.c:961: warning: expecting prototype for do not fallback to current(). Prototype was for get_mem_cgroup_from_current() instead"

Here kernel-doc parses the corresponding comment as a kernel-doc comment
and expects prototype for it in the next lines, and as a result causing
this warning.

Provide a clearer warning message to the users regarding the same, if the
content inside the comment does not follow the kernel-doc expected format.
Signed-off-by: Aditya Srivastava <yashsri421@gmail.com>
Link: https://lore.kernel.org/r/20210329092945.13152-1-yashsri421@gmail.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

scripts/kernel-doc

@@ -2109,15 +2109,17 @@ sub process_name($$) {
 	}
     } elsif (/$doc_decl/o) {
 	$identifier = $1;
-	if (/\s*([\w\s]+?)(\(\))?\s*([-:].*)?$/) {
+	my $is_kernel_comment = 0;
+	if (/^\s*\*\s*([\w\s]+?)(\(\))?\s*([-:].*)?$/) {
 	    $identifier = $1;
+	    $decl_type = 'function';
+	    $identifier =~ s/^define\s+//;
+	    $is_kernel_comment = 1;
 	}
 	if ($identifier =~ m/^(struct|union|enum|typedef)\b\s*(\S*)/) {
 	    $decl_type = $1;
 	    $identifier = $2;
-	} else {
-	    $decl_type = 'function';
-	    $identifier =~ s/^define\s+//;
+	    $is_kernel_comment = 1;
 	}
 	$identifier =~ s/\s+$//;
 
@@ -2139,6 +2141,13 @@ sub process_name($$) {
 	    $declaration_purpose = "";
 	}
 
+	if (!$is_kernel_comment) {
+	    print STDERR "${file}:$.: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst\n";
+	    print STDERR $_;
+	    ++$warnings;
+	    $state = STATE_NORMAL;
+	}
+
 	if (($declaration_purpose eq "") && $verbose) {
 	    print STDERR "${file}:$.: warning: missing initial short description on line:\n";
 	    print STDERR $_;