docs: add XML documentation for CMS SignedData stream generator (Batch 13a)

[KonradSop]

Description

Adds XML documentation to CmsSignedDataStreamGenerator — the streaming entry point for creating CMS SignedData (PKCS#7 signed-data) messages, the counterpart to CmsSignedDataGenerator (Batch 12a-3, #686):

• Class summary — describes the streaming Open/write/close workflow, the relationship to CmsSignedGenerator, and a usage example.
• Stream-handling note — closing the returned stream finalizes the CMS structure (certificates, CRLs, signer infos) but does not close the underlying stream; callers close it separately.
• Constructors — default and SecureRandom overload.
• SetBufferSize — documents the buffer length for the encapsulated content OCTET STRING segments.
• AddDigests overloads — both params string[] and IEnumerable<string>.
• AddSigner overloads — all eight families documented (certificate vs subject key id; default attributes vs AttributeTable vs CmsAttributeTableGenerator; inferred vs explicit encryption OID).
• Open overloads — documents encapsulated vs non-encapsulated content, explicit content type OID, and the optional data-output copy stream; <exception> tags on the core overload where it validates the supplied streams.

Legacy /** block comments converted to XML.

Key Accomplishments

• CMS streaming discoverability: IDE tooltips now cover the full public stream-generator surface used with AddCertificates / AddCrls from the base class.
• Accurate exception contracts: ArgumentNullException / ArgumentException documented only on the Open overload that validates its stream arguments.
• Consistent style: matches Batch 12a documentation conventions (1 → 22 <summary> blocks).
• No overlap with merged work: single file, disjoint from CmsSignedDataGenerator (docs: add XML documentation for CMS SignedData generator (Batch 12a-3) #686).

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.