bug#10771: TAB completion for info-display-manual

[Juri Linkov]

> The purpose of this command is to assist with switching between
> manuals that are already loaded, not in adding others.

If you often switch between loaded manuals, then this is easy to do
using the minibuffer history commands, e.g. after visiting two manuals:

M-x info-display-manual RET gnus RET
M-x info-display-manual RET emacs RET

switching between these two manuals:

M-x info-display-manual RET M-p M-p RET
M-x info-display-manual RET M-p M-p RET

BTW, do you know that an easy way to switch between manuals is using

`C-1 C-h i'
`C-2 C-h i'
`C-3 C-h i'
etc.

This is similar to switching between numbered frames
with e.g. `C-2 C-x 5 o'.

Do you switch them using frame numbers or frame names
with the command `select-frame-by-name'?

Maybe then `info-display-manual' should be renamed to
`info-select-manual-by-name'?