docs: add XML documentation for CMS AuthenticatedData stream generator (Batch 13c-2)

[KonradSop]

Description

Adds XML documentation to CmsAuthenticatedDataStreamGenerator — the streaming generator for CMS AuthenticatedData (MAC-protected) messages:

• Class summary — describes the recipient/Open/write/close workflow and a usage example.
• Stream-handling note — closing the returned stream finalizes the CMS structure and emits the MAC, but does not close the underlying stream.
• Constructors — default and SecureRandom overload.
• SetBufferSize and SetBerEncodeRecipients (BER vs DER recipient SET).
• Open overloads — the default-strength and explicit-key-size public overloads, plus the protected overload taking a pre-built MAC algorithm identifier; <exception cref="CmsException"/> where the implementation wraps MAC/parameter failures.

Legacy /** block comments converted to XML (public surface).

Key Accomplishments

• CMS authentication discoverability: IDE tooltips now cover the public/protected generator surface (1 → 8 <summary> blocks).
• Accurate exception contract: CmsException documented only on the Open overload that wraps failures.
• Consistent style: matches Batch 12a/13a documentation conventions.

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.