docs: add XML documentation for CMS SignedData generator (Batch 12a-3)

[KonradSop]

Description

Adds XML documentation to CmsSignedDataGenerator — the primary entry point for creating CMS
SignedData (PKCS#7 signed-data) messages:

• Class summary — describes the Generate workflow and relationship to CmsSignedGenerator.
• Constructors — default and SecureRandom overload.
• AddSigner overloads — all four families documented (certificate vs subject key id; default
  attributes vs AttributeTable vs CmsAttributeTableGenerator; inferred vs explicit encryption OID).
• AddSignerInfoGenerator — documents the SignerInfoGenerator integration path.
• Generate overloads — documents encapsulated vs detached content and explicit content type OID;
  <exception cref="CmsException"/> on paths that catch and rethrow signing/encoding failures.
• GenerateCounterSigners — documents counter-signature generation.
• UseDefiniteLength — documents DL encoding preference.

Legacy /** block comments converted to XML. One pre-existing commented line wrapped to ≤120 characters.

Key Accomplishments

• CMS signing discoverability: IDE tooltips now cover the full public generator surface used with
  AddCertificates / AddCrls from the base class.
• Accurate exception contracts: CmsException documented only on Generate and
  GenerateCounterSigners where the implementation wraps failures.
• Consistent style: matches Batch 12a-1/12a-2 documentation conventions.
• No overlap with open PRs: single file, disjoint from docs: add XML documentation for X.509 certificate and CRL generators (Batch 12a-1) #684 and 12a-2.

Verification

• Build Status: dotnet build crypto/src/BouncyCastle.Crypto.csproj -c Release — 0 errors, 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.