bug#38519: minibuffer-depth-indicate-mode with enable-recursive-minibuffers

[Juri Linkov]

A recent discussion about enable-recursive-minibuffers reminded
that minibuffer-depth-indicate-mode should be enabled when
enable-recursive-minibuffers is non-nil.  It's unusable
without the depth indication that is more error-prone.

It simultaneously enabling minibuffer-depth-indicate-mode
after enable-recursive-minibuffers becomes non-nil is not easy
to implement, then at least these two should mention each other
in the manual.