Update enhanced-parts/feature-specification.md (#4520)

* Update feature-specification.md

Remove configurable part directives.

General tweaks and rephrasings, and some typos.
Mention extension conflicts.
(You can break sub-parts by importing an extension
that causes a conflict. You can protect a part against that
by using explicit extension applications.)

* Update feature-specification.md

* Address comments.

* And without trailing whitespace or missed comments.

Author: lrhn