Document the project root cache and marker semantics

- New 'Project root cache' subsection in projects.adoc covering
  invalidation, when negative results bite (the recurring 'I created
  .projectile and Projectile still doesn't see it' confusion from
  #1936), and how to reset programmatically.
- Note in 'Customizing Project Detection' that
  projectile-root-top-down only matches regular files while
  projectile-root-bottom-up matches both - this difference is what
  forces VCS markers like .git onto the bottom-up list.
- Expand the file-local override section with a concrete example and
  call out that overrides bypass the cache.
- Rewrite projectile-project-root's docstring to mention caching,
  the tramp-archive unwrap, and the disconnected-remote behaviour
  that callers used to have to read the source to discover.

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