docs: add XML documentation for CMS AuthenticatedData (Batch 15c)

[KonradSop]

Description

Adds and completes XML documentation on CmsAuthenticatedData — the read-side entry point for CMS
AuthenticatedData (MAC-protected) messages:

• Class summary — decrypt and MAC verification workflow via GetRecipientInfos, RecipientID,
  RecipientInformation, and GetMac.
• Constructors — byte array, stream, and ContentInfo overloads; exceptions documented on the
  core constructor where validation or parsing fails (including the current
  NotImplementedException when authenticated attributes are present).
• Properties and accessors — MAC algorithm, originator information, recipient store, attribute
  tables, content digest helper, and DER encoding.
• Obsolete helpers — corrected the GetUnauthAttrs obsolete message to reference
  UnauthAttributes (was incorrectly pointing at AuthAttributes).

Legacy /** block comments converted to XML.

Key Accomplishments

• Authenticated-data discoverability: IDE tooltips now cover the full public read-side surface.
• Accurate exception contracts: documented only where the implementation throws.
• Focused scope: single file; no behavioural or signature changes beyond the obsolete-message fix.
• Documentation quality: all new or modified XML documentation lines are at most 120 characters.

Verification

• Build Status: dotnet build crypto/src/BouncyCastle.Crypto.csproj -c Release — 0 errors, no new warnings.
• Scope: Documentation-only except the incorrect obsolete attribute message on GetUnauthAttrs.

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.