improve template customisation documentation

[mirpedrol]

Apply suggestions from #1804

PR checklist

• [x] This comment contains a description of changes (with reason)
• [x] CHANGELOG.md is updated
• [ ] If you've fixed a bug or added code that should be tested, add tests!
• [x] Documentation in docs is updated

[mirpedrol]

@nf-core-bot fix linting

[codecov[bot]]

Codecov Report

Merging #1821 (8119253) into dev (19e0373) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev    #1821   +/-   ##
=======================================
  Coverage   61.10%   61.10%           
=======================================
  Files          37       37           
  Lines        4677     4677           
=======================================
  Hits         2858     2858           
  Misses       1819     1819           

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

[mirpedrol]

Thanks! I will need a ✅ from @ewels for the requested changes, or dismiss the review 😬