[backport#14161] doc/descriptors.md tweaks

Summary:
eeeaa29214 descriptors.md: Refer to descriptors as describing instead of matching (Russell Yanofsky)
eb49412562 doc/descriptors.md tweaks (Russell Yanofsky)

Pull request description:

Add some implementation details, and tweak phrasing in examples section to be more explicit about how expressions are used for matching.

Backport of Core PR14161

Test Plan:

read it

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7828