docs: add XML documentation for ML-KEM and ML-DSA parameter gaps (Batch 9)

[KonradSop]

Closes the documentation gaps left by PR #673 (Batch 5, ML-KEM and ML-DSA initial pass). PR #673 covered MLKemParameters, MLKemPublicKeyParameters, MLKemPrivateKeyParameters, MLDsaParameters and MLDsaPublicKeyParameters; this PR documents the remaining seven public types so the FIPS 203 / FIPS 204 surface is consistent.

• MLKemParameterSet — class summary citing FIPS 203; per-constant summaries naming each parameter set (NIST Category 1/3/5, module rank k); doc on EncapsulationLength, SecretLength, Name, ToString.
• MLKemKeyParameters — abstract base; class summary plus Parameters accessor doc.
• MLKemKeyGenerationParameters — class summary covering the SecureRandom + parameter-set pairing and the strength = 0 rationale; both constructors document their ArgumentNullException / ArgumentException contracts.
• MLDsaParameterSet — class summary citing FIPS 204; per-constant summaries (NIST Category 2/3/5, modes 2/3/5); doc on Name, ToString.
• MLDsaKeyParameters — abstract base; class summary plus Parameters accessor doc.
• MLDsaKeyGenerationParameters — same shape as the ML-KEM counterpart; OID-lookup constructor exception text notes both pure and HashML-DSA OIDs are accepted.
• MLDsaPrivateKeyParameters — class summary explaining the (ρ, K, tr, s₁, s₂, t₀) expansion plus cached t₁. Format enum documented per value (SeedOnly, EncodingOnly, SeedAndEncoding). FromEncoding, both FromSeed overloads, GetEncoded, GetPublicKey, GetPublicKeyEncoded, GetSeed, PreferredFormat, WithPreferredFormat documented with their full exception contracts.

Key Accomplishments

• Closes Batch 5 holes: completes the FIPS 203 / FIPS 204 documentation surface that PR docs: add XML documentation for ML-KEM and ML-DSA parameters (Batch 5) #673 left half-finished. ML-KEM and ML-DSA now match the depth of coverage given to SLH-DSA in Batch 8.
• Accurate exception contracts: <exception> tags only added where the method body actually throws — ArgumentNullException / ArgumentException on FromEncoding, FromSeed and OID-lookup paths; InvalidOperationException on WithPreferredFormat when a seed is required but unavailable.
• Consistent style: Class summaries follow the Batch 5 / Batch 8 PQC-params pattern (FIPS reference, what is wrapped, decoding contract). Internal members and pre-existing implementation comments left untouched.
• No overlap with open PRs: scope is disjoint from feature/modern-curves-docs (Batch 7) and feature/slh-dsa-docs (Batch 8) — each of the seven files in this PR is untouched by either branch.

Verification

• Build Status: dotnet build crypto/src/BouncyCastle.Crypto.csproj -c Release — succeeded on net6.0, netstandard2.0, net461 with no new warnings.
• Scope: Documentation-only; no behavioural or signature changes.

Checklist before requesting a review

• I have performed a self-review of my code
• I have kept the patch limited to only change the parts related to the patch
• This change requires a documentation update

See also Contributing Guidelines.