[chore] Tail-sampling drop policy documentation (open-telemetry#39801)

Documentation for the recently added [drop policy
type](open-telemetry#39668).
The examples replace the use of invert_match. I attempted to make as few
changes as necessary at this time. I intended to add further
documentation while deprecating the top-level invert_match
logic/decisions in a separate pull-request.

Signed-off-by: Sean Porter <[email protected]>
# Conflicts:
#	processor/tailsamplingprocessor/README.md

Author: portertech