Move the documentation for the "poll" command up with

Move the documentation for the "poll" command up with
other server configuration.  Explain what it's about;
reference the related "$target_name curstate" method.

Update "poll" output to report whether background polling
is enabled or not.

Also fix a small typo; PC's have "complementary" tools.
Some have also "complimentary" ones; but not all.

git-svn-id: svn://svn.berlios.de/openocd/trunk@2178 b42882b7-edfa-0310-969c-e2dbd0fdcd60