gdb/doc: improve 'show print elements' description

The documentation for 'show print elements' contains the line:

  If the number is 0, then the printing is unlimited.

However, this line is now out of date as can be seen by this GDB
session:

  (gdb) set print elements 0
  (gdb) show print elements
  Limit on string chars or array elements to print is unlimited.

The value 0 does indeed mean unlimited, and this is described in the
'set print elements' section, however, for 'show print elements' the
user will never see the value 0, so lets just remove that bit from the
docs.