doc clarifications for server flags

The "-f" is a shortcut for "-c" ... and providing any "-c" options
means the "openocd.cfg" file isn't implicitly used.  Both the User's
Guide and the manual page were weak on these points, which has led
to some confusion.

Also update the manual page to include highlights of the search path
mechanism, including the facts that it exists and that "-s" adds to it.
Stop saying only the current directory is involved; the OpenOCD
script library is quite significant.

(Missing: complete manpage coverage of the search path, including a
FILES section listing all components and saying where the script
library is found.)

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>