runtime(java): Optionally highlight the :: token for method references

This token will be highlighted, similar to the arrow of
lambda expressions, whenever "g:java_highlight_functions" is
defined.

Also:

- Improve the recognition of _switch-case_ labels
  (D-Cysteine).
- Remove insignificant empty statements in syntax test
  files.

closes: #15322

References:
https://docs.oracle.com/javase/specs/jls/se21/html/jls-15.html#jls-15.13
https://github.com/fleiner/vim/pull/1

Co-authored-by: D-Cysteine <54219287+D-Cysteine@users.noreply.github.com>
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 2dfc5b6..f843082 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 Jul 16
+*syntax.txt*	For Vim version 9.1.  Last change: 2024 Jul 23
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -2046,11 +2046,11 @@
 	:let java_highlight_java_io=1
 Check the javaid.vim file for a list of all the packages that are supported.
 
-Function names are not highlighted, as the way to find functions depends on
-how you write Java code.  The syntax file knows two possible ways to highlight
-headers of function declarations:
+Headers of indented function declarations can be highlighted (along with parts
+of lambda expressions and method reference expressions), but it depends on how
+you write Java code.  Two formats are recognized:
 
-If you write function declarations that are consistently indented by either
+1) If you write function declarations that are consistently indented by either
 a tab, or a space . . . or eight space character(s), you may want to set >
 	:let java_highlight_functions="indent"
 	:let java_highlight_functions="indent1"
@@ -2062,10 +2062,12 @@
 	:let java_highlight_functions="indent7"
 	:let java_highlight_functions="indent8"
 Note that in terms of 'shiftwidth', this is the leftmost step of indentation.
-However, if you follow the Java guidelines about how functions and classes are
-supposed to be named (with respect to upper- and lowercase) and there is any
-amount of indentation, you may want to set >
+
+2) However, if you follow the Java guidelines about how functions and types
+are supposed to be named (with respect to upper- and lowercase) and there is
+any amount of indentation, you may want to set >
 	:let java_highlight_functions="style"
+
 In addition, you can combine any value of "java_highlight_functions" with >
 	:let java_highlight_signature=1
 to have the name of a function with its parameter list parens distinctly