docs: add XML documentation for CMS SignedData stream parser (Batch 15g)

[KonradSop]

Description

Adds XML documentation to CmsSignedDataParser — the streaming read-side counterpart to
CmsSignedData (Batch 12a-3, #686) and the parser-side pair to CmsSignedDataStreamGenerator
(Batch 13a, #687):

• Class summary — describes the streaming verify workflow, the one-signer-per-pass constraint,
  and the requirement to drain encapsulated content before calling GetSignerInfos.
• <remarks> — stream lifecycle (parser does not close the input stream; dispose the parser or
  close manually), no internal buffering, and a C# example covering drain → certificates → verify.
• Constructors — all four overloads (byte[] / stream, with optional detached CmsTypedStream);
  <exception> tags on the core (CmsTypedStream, Stream) constructor.
• Public API — Version, DigestOids, GetSignerInfos, certificate/CRL/revocation stores,
  SignedContentType, GetSignedContent, and static ReplaceSigners /
  ReplaceCertificatesAndCrls.
• Cross-references — CmsSignedData, CmsSignedDataStreamGenerator, SignerInformation,
  SignerInformationStore, CmsTypedStream.Drain.

Legacy /** block comments converted to XML.

Key Accomplishments

• CMS streaming discoverability: IDE tooltips now cover the full public parser surface used to
  verify CMS SignedData from a stream without loading the entire message into memory.
• Accurate workflow documentation: drain-before-GetSignerInfos ordering and digest finalization
  are documented with <see cref> links rather than leaving callers to infer from source.
• Consistent style: matches Batch 14–15 documentation conventions (class summary + workflow +
  <exception> only where the code throws).
• No overlap with merged or open work: single file, disjoint from CmsSignedData (docs: add XML documentation for CMS SignedData generator (Batch 12a-3) #686),
  CmsSignedDataStreamGenerator (docs: add XML documentation for CMS SignedData stream generator (Batch 13a) #687), and signer types (docs: add XML documentation for CMS signer types (Batch 14b) #699).

Verification

• Build Status: dotnet build crypto/src/BouncyCastle.Crypto.csproj -c Release — 0 errors, no new warnings.
• Line length: all new or modified /// lines are at most 120 characters.
• 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.