Issue #1558: Clarify documentation for the --ignore option

[MatthiasZepper]

• This comment contains a description of changes (with reason)

This pull request updates docs/usage.md to clarify the section on the --ignore option. The documentation now also briefly mentions the --ignore-symlinks flag, which was introduced in MultiQC v1.6, but not yet covered.

• CHANGELOG.md has been updated

Not applicable

[ewels]

Thanks! Sorry, giving you a docs task was a bit mean as I have my own idiosyncrasies that I usually stick to..

• One sentence per line
  • GitHub inserts newlines after a single line break, but most markdown rendering engines (including the MultiQC and nf-core websites) ignore them.
  • Having shorter lines makes the diffs easier to read when editing docs
  • Using sentences as natural line breaks (rather than a character width) makes editing easier and diffs clearer.
• I usually draw attention to "gotchas" like the quotes thing here with blockquotes, then use CSS to make these highlighted. This is mostly because Markdown doesn't have (easy) ways to have custom styles for this like RST (though there are extensions). It works fine.

Otherwise great!