docs: publish the FULL surface in XFF.md, calling out what a build may lack

[helly25]

XFF.md was generated with --config=xff_full, which enables only PCRE2 - so the archive flags appeared as "NOT built into this binary" instead of being documented. The published reference should describe the tool, with a clear callout of what a given build might not include.

Two parts, because the config alone was not enough:

• --config=xff_docs - every composable extra on, used by xff-md-update.sh and the CI drift guard so the two cannot diverge. Add each new extra here as it lands.
• BuildReference(Audience) - the per-binary "NOT built into this binary" line is a property of the binary that generated the document, not of the feature. --markdown now renders Audience::kPublished and omits it; --help / --man keep Audience::kThisBinary, where the note is exactly right because it IS about your build. This part is necessary because the archive extra has no linkable backend yet, so enabling its flag cannot make the note disappear on its own.

The reader is still told what they may not get, by the STATIC text that survives in both audiences:

A build-time extra: the stock binary is lean and omits it (rebuild with --//xff:xff_archive);
  asking for archive handling without it is a hard error.
A build-time extra: present only in a full build - run `xff --help=extras` to see whether THIS
  binary has it.

XFF.md now carries zero per-binary notes.

Three tests pin the split: a published reference has no such note; a this-binary reference does (asserted against the same ExtraEnabled predicate the builder uses, so it holds whichever extras the test binary links); and both audiences still name the rebuild flag - that last one is what keeps the callout from being silently lost later.

Verified: bazel test //... green, //xff/cli:xff_markdown_test --config=xff_docs green, hooks clean.

[helly25]

Closing: this was pushed under a new branch name because a rebase had diverged from what was already pushed. Reopening the same work from its original branch (docs/xff-md-all-extras), updated by MERGING main rather than rebasing, so no force-push is needed.