Minor: format comments in `PushDownFilter` rule

[alamb]

Which issue does this PR close?

Related to https://github.com/apache/datafusion/issues/10291

Rationale for this change

While reviewing this code for https://github.com/apache/datafusion/issues/10291 I found several places the doc comments could be improved, and I thought those changes would be easy to review as their own PR

What changes are included in this PR?

• make // to ///
• Add some additional markdown style

Are these changes tested?

Are there any user-facing changes?

No this is internal documentation -- none of these changes are user facing