The implicit "any unprefixed line is an ignore pattern" rule is the
last source of subtle parser surprises — it's the reason why a
single leading space silently changes a +-keep into a literal ignore
pattern, and it makes typo'd comments slip through as ignores.

Mark these lines as :legacy-ignore in the classifier, record them
in a new prefixless-ignore slot on the dirconfig struct, and emit a
one-time warning per project listing the offending entries. The
behavior is unchanged — the lines still go into the ignore list —
but users now get a nudge to write them as -entry. The warning can
be silenced via projectile-warn-on-prefixless-dirconfig-lines.

The parser docstring only described + and - prefixes even though the
function returns a 3-tuple including the ! ensure bucket. Fill that
in, and pull the path-vs-glob distinction out of the prose in
projects.adoc into a subsection of its own — that ambiguity is at
the root of recurring confusion (#740, #1109, #680, #1941).

- VCS commands: table of all per-VCS command variables, plus
  projectile-git-submodule-command and projectile-git-ignored-command
- Caching: projectile-files-cache-expire
- Tags: projectile-tags-file-name, projectile-tags-backend
- Searching: projectile-use-git-grep
- Project name: projectile-project-name-function
- Dirty projects: projectile-vcs-dirty-state
- Hooks: projectile-find-file-hook, projectile-find-dir-hook,
  projectile-before-switch-project-hook
- Misc: projectile-verbose, projectile-show-menu
- Ignore/unignore: projectile-globally-ignored-file-suffixes,
  projectile-global-ignore-file-patterns,
  projectile-globally-unignored-files,
  projectile-globally-unignored-directories

- fd integration: projectile-git-use-fd, projectile-fd-executable,
  projectile-git-fd-args
- Known projects: projectile-known-projects-file,
  projectile-ignored-projects, projectile-ignored-project-function,
  projectile-track-known-projects-automatically
- Buffer filtering: projectile-buffers-filter-function,
  projectile-kill-buffers-filter
- Test/impl toggling: projectile-default-src-directory,
  projectile-default-test-directory, projectile-test-prefix-function,
  projectile-test-suffix-function, projectile-create-missing-test-files

Fixes #1954

That's a safer default, given the changes to its behavior in 2.9.