Close pr/32278, my last submitted PR as a non-committer.

Add descriptions of what a port distfile is, and how it's used. Use
the word distfile instead of "the program source code.

Not certain if this commit message is formatted correctly: the
X-forwarded emacs doesn't present the standard commit template. Must
ask my mentor why not, but don't want to leave this hanging.

PR: docs/32278