gdb, doc: correct argument description for info connections/inferiors

It said for 'info inferiors' and 'info connections' that the argument
could be 'a space separated list of inferior numbers' which is correct
but incomplete.  In fact the arguments can be any space separated
combination of numbers and (ascending) ranges.

The beginning of the section now describes the ID list as a new keyword.

Co-Authored-By: Christina Schimpe <christina.schimpe@intel.com>
1 file changed