docs: float placement and two columns are defaults now, not opt-in flags

[tannevaled]

The docs still described two subsystems as gated and off by default, which understated the engine. Both are opt-out in the source:

floatsEnabled()  = os.Getenv("GOTEX_FLOATS")    != "0"
twoColumnOptIn() = os.Getenv("GOTEX_TWOCOLUMN") != "0"

Real float placement became the default in #201, and the classes that set two columns themselves (revtex reprint, acmart sigconf/sigplan, IEEEtran) in #200, #218 and #220/#221 — the last of which covers a paper that bundles acmart.cls.

Corrected in three places

• The flag table, whose heading claimed every flag was "off by default". The two opt-out flags now appear in their =0 disabling form (the convention GOTEX_BEAMER=0 already used in that same table), and the genuinely opt-in ones are marked as such.
• The two "Known gaps" bullets, rewritten to name what is actually still missing rather than a gate that no longer exists: that a figure's true geometry does not yet pay for itself (giving un-rasterisable figures their real aspect ratio leaves total page error unchanged, improving only the systematic bias), that the page builder has no insertions and no user \output — which is what real float and footnote placement is built on — and that full-width figure*/table* spanning bands are the remaining two-column rough edge.
• The revtex4 row, which said the reprint layout needed GOTEX_TWOCOLUMN.

Hugo builds clean. Documentation only, no behaviour change.

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