docs: correct the bacpac claims, and document --schema-only

[JeePeeTee]

Closes #23 and #24, both of which came out of the r/SQLServer thread.

#23 — the bacpac claims were wrong

Two commenters pushed back on the same passage, independently, and both were right.

Users and roles. A bacpac carries database users and database roles — not "contained users at
best". What it does not carry is the server logins behind them, so they restore orphaned. That
is a real limitation and a more useful thing to tell the reader.

The error is worth naming precisely, because it is a reasoning failure rather than a typo: the
claim came from opening one database's model.xml, finding no user or role elements, and writing
that up as what the format does. One sample is not a specification. The corrected passage now says
what was measured, and what that does and does not license.

"All-or-nothing". Also too strong — sqlpackage /p:TableData= selects which tables' data to
include. What survives is narrower and still true: no row filter within a table, and the schema is
always complete. Corrected in the README and in docs/dbdumper-vs-dacfx.md.

One more, found while auditing. When not to use this said a server-side bacpac is preferable
"not because a bacpac holds more" — that sentence is about artifact size (488 MB vs 498 MB), but
after the correction above it reads as a claim about coverage. Reworded to say "smaller".

#24 — --schema-only existed but was invisible

It has worked since the first commit and skips the data path entirely, but it appeared only as a
row in the export flag table. #19 was filed asking for the feature — by someone who had read the
README. That is about as clear a signal as documentation feedback gets.

It now has a section next to the other filtering features, covering the nightly-schema-into-git job
that motivated the request, and two caveats a reader deserves up front:

• output is not byte-identical between runs yet (Deterministic export output between runs #21), so a nightly commit can show changes when
  nothing moved
• a schema-only archive restores as empty tables, and verify then compares zero against zero and
  reports OK — correct, but surprising

Cross-linked from When it is the right tool.

Checks

• One claim in my own first draft was wrong and got fixed before commit: I wrote that
  --schema-only writes no work directory. It does — openSpool runs before the SchemaOnly
  check, so a .part directory is created and then discarded. Verified by running it: the archive
  has 0 data/ entries and nothing is left behind, but the work directory does exist during
  the run. The text now says what actually happens.
• Every internal anchor re-validated: 4 links against 27 headings, none broken.
• No real server or database names introduced — checked against the list this repo has previously
  had scrubbed from it.
• Documentation only; no code touched.