patch 9.1.1600: using diff anchors with hidden buffers fails silently

Problem:  diff: using diff anchors with hidden buffers fails silently
Solution: Give specific error message for diff anchors when using hidden
          buffers (Yee Cheng Chin).

Diff anchors currently will fail to parse if a buffer used for diff'ing
is hidden. Previously it would just fail as the code assumes it would
not happen normally, but this is actually possible to do if `closeoff`
and `hideoff` are not set in diffopt. Git's default diff tool "vimdiff3"
also takes advantage of this.

This fix this properly would require the `{address}` parser to be
smarter about whether a particular address relies on window position or
not (e.g. the `'.` address requires an active window, but `'a` or `1234`
do not). Since hidden diff buffers seem relatively niche, just provide a
better error message / documentation for now. This could be improved
later if there's a demand for it.

related: #17615
closes: #17904

Signed-off-by: Yee Cheng Chin <ychin.git@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/doc/options.txt

@@ -1,4 +1,4 @@
-*options.txt*	For Vim version 9.1.  Last change: 2025 Aug 06
+*options.txt*	For Vim version 9.1.  Last change: 2025 Aug 07
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -3016,6 +3016,8 @@ A jump table for the options with a short description can be found at |Q_op|.
 	If some of the {address} do not resolve to a line in each buffer (e.g.
 	a pattern search that does not match anything), none of the anchors
 	will be used.
+								*E1562*
+	Diff anchors can only be used when there are no hidden diff buffers.
 
 						*'dex'* *'diffexpr'*
 'diffexpr' 'dex'	string	(default "")

runtime/doc/tags

@@ -4694,6 +4694,7 @@ E1559	vim9.txt	/*E1559*
 E156	sign.txt	/*E156*
 E1560	vim9.txt	/*E1560*
 E1561	vim9.txt	/*E1561*
+E1562	options.txt	/*E1562*
 E157	sign.txt	/*E157*
 E158	sign.txt	/*E158*
 E159	sign.txt	/*E159*