Bring Syntax.md up to date with the functions that now exist

[Rafael-SOWNet]

Syntax.md still listed floor, ceil, ceiling, round, min, max and gcd under Refused by name. Six of them have existed since #827 and #828, so the document told a caller the opposite of what the parser does.

AGENTS.md is explicit that this file has to be kept true when the grammar changes, and that a stale one is worse than none. I changed the grammar twice and did not update it either time — this is that debt.

The new entries say what each function does, not merely that it parses, because the conventions are the part a caller cannot guess:

• floor and ceil round toward the infinities, not toward zero — floor(-3/2) is -2
• round goes to the nearest even on a tie, so round(1/2) is 0 and round(5/2) is 2, and it is therefore not floor(x + 1/2)
• all three take a complex argument componentwise
• min/max/gcd are variadic and fold; min and max leave an unordered pair as written
• gcd covers rationals — gcd(1/2, 1/3) is 1/6 — and leaves the polynomial case alone

The refused list is now just trunc, lcm, erf and conjugate.

Every claim was checked against a build, not written from memory: the seven values, that ceiling(x) prints back as ceil(x), that min(x, y) and gcd(x, y) stay as written, that the four remaining names still raise, and that re/im are still read as products.

Docs only.

[Rafael-SOWNet]

Added a second commit to the same file. Syntax.md repeated the claim #823 corrected in AGENTS.md — "nothing parses LaTeX" — and I had fixed only the copy I was pointed at. CSharpMath.Evaluation reads LaTeX back into an Entity and states the contract in its own source, so the sentence was wrong in both places.

The correction keeps the useful half: Latexise is still free to use \frac and the rest, but changing what it emits can break a downstream project and nothing in this repository will catch it.