Document remaining indexing semantics

Three doc gaps surfaced during the indexing review:

* The `*' prefix on `projectile-globally-ignored-directories' entries
  is not a glob - it is what promotes a basename match from
  "anchored at the project root" to "anywhere in the tree", and it
  only takes effect under hybrid indexing.
* The default `projectile-generic-command' picks `fd' when it's on
  PATH and falls back to a `find ... | tr ...' pipeline. The
  fallback does not exclude common build directories, which is a
  trap for non-VCS projects under alien on hosts without `fd'.
* `projectile-git-use-fd' controls how Projectile handles
  deleted-but-unstaged files: with `fd' they disappear immediately,
  with `git ls-files' Projectile post-filters via `git ls-files -zd'.

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