style.mdoc: Remove synopsis formatting advice

Remove incorrect advice. The first rule was no spaces padding pipes
when showing alternates. Almost universally we not do this, and third
party manuals do not either. The second was using Cm to mark up symbols.
This advice is in conflict with mdoc(7), and the extended documentation
on mdoc linked below, which is also linked at the bottom of mdoc(7).

Conflicting information in our docs is a natural result of documentation
proliferation. The most important thing we can do to reduce technical
debt in the documentation is reduce the amount of doc overlap. Excessive
technical debt and conflicting rules is a barrier to new contributors.
This type of work requires thorough expertise, is not glamorous at all,
and to add insult to injury, is socially unsafe. To get new contributors
to the docs project, we must deduplicate documentation.

We have the mdoc manual, examples/mdoc, style.mdoc, and the fdp chapter
on manual pages. With this many, they will inevitably come to contain
conflicting information, and people will learn not to read them or work
on them.

MFC after: 3 days
Ref: https://mandoc.bsd.lv/mdoc/style/commands.html
Differential Revision: https://reviews.freebsd.org/D55298

(cherry picked from commit 47326cb470278e891975354286481b5ba876929a)