gdb/doc: make use of group/end group in 'info pretty-printers' example

gdb/doc: make use of group/end group in 'info pretty-printers' example

The 'info pretty-printers' example is pretty long and consists of many
commands and their output.

Currently, when the pdf manual is generated this example spans a
page-break, with the page-break falling part way through some example
output from GDB.

This commit breaks up the example using @group .... @end group, within
each group is a single GDB command and all its output.

Now, when the pdf manual is created, the page-break is placed after
the output of one GDB command, and before the subsequent command, this
looks much nicer.