manual: Fix ld.so diagnostics menu/section structure

And shorten the section/node names a bit, so that the menu
entries become easier to read.

Texinfo 6.5 fails to process the previous structure:

./dynlink.texi:56: warning: node `Dynamic Linker Introspection' is
  next for `Dynamic Linker Diagnostics' in sectioning but not in menu
./dynlink.texi:56: warning: node up `Dynamic Linker Diagnostics'
  in menu `Dynamic Linker Invocation' and
  in sectioning `Dynamic Linker' differ
./dynlink.texi:1: node `Dynamic Linker' lacks menu item for
  `Dynamic Linker Diagnostics' despite being its Up target
./dynlink.texi:226: warning: node prev `Dynamic Linker Introspection' in menu `Dynamic Linker Invocation'
  and in sectioning `Dynamic Linker Diagnostics' differ

Texinfo 7.0.2 does not report an error.

This fixes commit f21962dd
("manual: Document ld.so --list-diagnostics output").

Reviewed-by: Szabolcs Nagy <szabolcs.nagy@arm.com>