Document how resolveId is cached

[lukastaegert]

This PR contains:

• bugfix
• feature
• refactor
• documentation
• other

Are tests included?

• yes (bugfixes and features will not be merged without tests)
• no

Breaking Changes?

• yes (breaking changes will not be merged unless absolutely necessary)
• no

List any relevant issue numbers:

Description

When working on caching the commonjs plugin, I found that it is not documented that caching will not only skip the load hook but also the resolveId hook of dependencies (which does indeed save a lot).

[github-actions]

Thank you for your contribution! ❤️

You can try out this pull request locally by installing Rollup via

npm install rollup/rollup#document-resolve-id-caching

or load it into the REPL:
https://rollupjs.org/repl/?pr=4407

[codecov]

Codecov Report

Merging #4407 (6c34232) into master (5bd96df) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4407   +/-   ##
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  Coverage   98.72%   98.72%           
=======================================
  Files         204      204           
  Lines        7319     7319           
  Branches     2081     2081           
=======================================
  Hits         7226     7226           
  Misses         34       34           
  Partials       59       59           

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