Clarify information on excluding results from searches

[felicitymay]

Code of Conduct

• [X] I have read and agree to the GitHub Docs project's Code of Conduct

What article on docs.github.com is affected?

Understanding the search syntax

What part(s) of the article would you like to see updated?

The existing section on "Exclude certain results" is misleading because it starts with the NOT syntax first and this can be used only for string keywords. Information about how to negate qualifiers is easy to miss within this section (see https://github.com/github/docs/issues/22902).

Recommended changes

1. Create a new section called "Exclude results that match a qualifier" and move the information about using - into this section. The content will need a small amount of rewording for the new location. I can also see that the formatting of the table needs to be adjusted.
2. Rename the existing section to "Exclude results with specific keywords"

Additional information

This is a follow up to https://github.com/github/docs/issues/22902.