docs: add XML documentation for CMS recipient info generators (Batch 16c)

[KonradSop]

Description

Adds XML documentation to the CMS write-side recipient-info generators used when building
EnvelopedData messages via CmsEnvelopedGenerator (Batch 15a, #700):

• KeyTransRecipientInfoGenerator - class summary, three constructors, Generate, and protected
  wrap helpers; links to CmsEnvelopedGenerator.AddKeyTransRecipient and
  KeyTransRecipientInformation (Batch 14a, docs: add XML documentation for CMS recipient types (Batch 14a) #698).
• KeyAgreeRecipientInfoGenerator - internal key-agreement generator; configuration properties
  and Generate with <exception> tags where the implementation throws.
• KekRecipientInfoGenerator - internal KEK generator configured by AddKekRecipient.
• PasswordRecipientInfoGenerator - internal password-based generator configured by
  AddPasswordRecipient.

Key Accomplishments

• CMS write-side discoverability: IDE tooltips now cover all four RecipientInfoGenerator
  implementations used when adding enveloped-data recipients.
• Accurate exception contracts: documented only on KeyAgreeRecipientInfoGenerator.Generate
  where the code throws.
• Focused bundle: four related generator files only; 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.