Improve ProjectionExpr documentation and comments

[alamb]

Which issue does this PR close?

• related to https://github.com/apache/datafusion/pull/19111

Rationale for this change

While reviewing https://github.com/apache/datafusion/pull/19111 from @adriangb I found a few places that the documentation could be improved to give higher level context

What changes are included in this PR?

Update some docs and comments

Are these changes tested?

By cos

Are there any user-facing changes?

Just docs/comments. No functional changes