gdb/doc: extend the documentation of the jump command

gdb/doc: extend the documentation of the jump command

This commit addresses PR gdb/7946.  While checking for bugs relating
to the jump command I noticed a long standing bug that points out a
deficiency with GDB's documentation of the jump command.

The bug points out that 'jump 0x...' is not always the same as 'set
$pc = 0x...' and then 'continue'.  Writing directly to the $pc
register does not update any auxiliary state, e.g. $npc on SPARC,
while using 'jump' does.

It felt like this would be an easy issue to address by adding a
paragraph to the docs, so I took a stab at writing something suitable.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=7946

Approved-By: Eli Zaretskii <eliz@gnu.org>