docs: add XML documentation for CMS EnvelopedData stream parser (Batch 15b)

[KonradSop]

Description

Adds XML documentation to CmsEnvelopedDataParser — the streaming read-side counterpart to
CmsEnvelopedData (Batch 15a, #700):

• Class summary — streaming decrypt workflow, single-recipient constraint, and call-order note.
• Remarks — lazy encrypted-content draining via RecipientInformation, stream-disposal note,
  buffering guidance, and a C# usage example.
• Constructors — byte array and stream overloads; ArgumentNullException and CmsException
  documented on the stream constructor where parsing validates input.
• Properties and accessors — content-encryption algorithm, originator information, recipient
  store, and unprotected attributes.

Legacy /** block comments converted to XML.

Key Accomplishments

• Streaming enveloped-data discoverability: IDE tooltips now cover the parser surface used for
  large-message decrypt workflows.
• Accurate exception contracts: documented only where the constructor validates or parses input.
• Focused scope: single file; 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, 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.