docs: add XML documentation for CMS recipient types (Batch 14a)

[KonradSop]

Description

Adds XML documentation to the CMS recipient-information API used to identify
recipients and recover encrypted CMS content:

• RecipientInformation — base-class workflow, recipient identifier,
  key-encryption algorithm, complete-content convenience method, and content
  stream method.
• RecipientId — recipient matching by issuer/serial number, subject key
  identifier, or KEK key identifier; documents equality semantics.
• Key transport, key agreement, KEK, and password recipients — class
  summaries and GetContentStream contracts, including required key material,
  returned stream, and implementation-backed exceptions.

A separate preceding commit performs whitespace-only normalization in
RecipientId.cs and KEKRecipientInformation.cs, as required by the
repository contribution rules.

Key Accomplishments

• CMS recipient discoverability: IDE tooltips now explain how to select a
  recipient and provide the appropriate private, shared, or password-based key.
• Accurate contracts: stream-return and exception documentation follows
  the actual key-recovery implementations.
• Focused scope: six CMS recipient API files only; no behavioural or
  signature changes.
• 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.
• Review: independent diff review completed; one RecipientId description
  omission was corrected before commit.
• Scope: documentation and a separate mechanical whitespace cleanup only.

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.