bug#53478: 26.3; Document text-terminal (non)support of pixel-relevant features

[Drew Adams]

I tried to help with this user question:

https://emacs.stackexchange.com/q/70231/105

I'm no expert on this.  I guessed that Emacs in terminal mode can't
support most pixel-level operations.

I wanted to point the user to the Emacs manual about this.  I tried to
find info about it with `i' using keyword `terminal' (even with
substring matching for completions).  I found nothing.  Likewise, with
keyword `pixel'.

I then used `C-s' to search the manual for `terminal', with no luck.  I
then tried searching the manual for `pixel'.  The only relevant info I
found was Glossary entry Text Terminal.

Assuming I just searched poorly, please consider adding some index
entries for this information.  If the info is missing, apart from that
caveat in the Glossary, please consider adding some relevant info.

In GNU Emacs 26.3 (build 1, x86_64-w64-mingw32)
 of 2019-08-29
Repository revision: 96dd0196c28bc36779584e47fffcca433c9309cd
Windowing system distributor `Microsoft Corp.', version 10.0.19043
Configured using:
 `configure --without-dbus --host=x86_64-w64-mingw32
 --without-compress-install 'CFLAGS=-O2 -static -g3''