runtime(java): Highlight javaConceptKind modifiers with StorageClass

Stop assigning by default the NonText highlighting group for
javaConceptKind modifiers since its colour is hardly
distinguishable from a background colour for a range of
colour schemes.

fixes #15237
related #15238
closes: #15664

Co-authored-by: Dexter Gaon-Shatford <dexter@gaonshatford.ca>
Signed-off-by: Aliaksei Budavei <0x000c70@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/doc/syntax.txt b/runtime/doc/syntax.txt
index 910aee5..7650433 100644
--- a/runtime/doc/syntax.txt
+++ b/runtime/doc/syntax.txt
@@ -1,4 +1,4 @@
-*syntax.txt*	For Vim version 9.1.  Last change: 2024 Aug 31
+*syntax.txt*	For Vim version 9.1.  Last change: 2024 Sep 11
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -2155,6 +2155,12 @@
 or >
 	:hi javaParen ctermfg=blue guifg=#0000ff
 
+Certain modifiers are incompatible with each other, e.g. `abstract` and
+`final`: >
+	:syn list javaConceptKind
+and can be differently highlighted as a group than other modifiers with >
+	:hi link javaConceptKind NonText
+
 If you notice highlighting errors while scrolling backwards, which are fixed
 when redrawing with CTRL-L, try setting the "g:java_minlines" variable to
 a larger number: >