vim-patch:9.1.1600: using diff anchors with hidden buffers fails silently (#35218)

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: vim/vim#17615
closes: vim/vim#17904

https://github.com/vim/vim/commit/cad3b2421de7b703e0ee619850a8a3bc55454281

Co-authored-by: Yee Cheng Chin <ychin.git@gmail.com>

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