docs: add XML documentation for CMS exceptions and PBE keys (Batch 16e)

[KonradSop]

Description

Adds XML documentation to CMS exception types and password-based key classes used with enveloped-data
recipients:

• CmsException and derived types - class summaries for CmsAttributeTableGenerationException,
  CmsStreamException, CmsAlgorithmNotAllowedException, CmsTagLengthException, and
  CmsVerifierCertificateNotValidException.
• CmsPbeKey - base class summary, constructors, properties, and GetEncoded; links to
  CmsEnvelopedGenerator.AddPasswordRecipient.
• Pkcs5Scheme2Utf8PbeKey / Pkcs5Scheme2PbeKey - UTF-8 vs ASCII password encoding; public
  constructors documented with <inheritdoc/>.

Legacy /** block comments converted to XML where present.

Key Accomplishments

• CMS error discoverability: IDE tooltips now identify CMS-specific exception types.
• PBE recipient clarity: documents password-based key material used with AddPasswordRecipient.
• Accurate exception contracts: <exception> only on CmsPbeKey constructors where PBKDF2 is validated.
• Focused bundle: nine related CMS support files; no behavioural or signature changes.
• Documentation quality: all new or modified /// 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; 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.