docs: add XML documentation for CMS EnvelopedData stream generator (Batch 13c-3)

[KonradSop]

Description

Adds XML documentation to CmsEnvelopedDataStreamGenerator — the streaming generator for CMS EnvelopedData (PKCS#7 enveloped-data) messages:

• Class summary — describes the recipient/Open/write/close workflow and a usage example.
• Stream-handling note — closing the returned stream finalizes the CMS structure but does not close the underlying stream; includes the caveat that a buffering encoder stream left unclosed can truncate the encoded output.
• Constructors — default and SecureRandom overload.
• SetBufferSize and SetBerEncodeRecipients (BER vs DER recipient SET).
• Open overloads — the default-strength and explicit-key-size public overloads.

Legacy /** block comments converted to XML; existing terse summaries kept consistent with siblings.

Key Accomplishments

• CMS enveloping discoverability: IDE tooltips now cover the full public generator surface (4 → 8 <summary> blocks).
• Consistent style: matches Batch 12a/13a conventions and the CMS stream siblings (13c-1/13c-2).
• No overlap: single file, disjoint from the other CMS stream generators.

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.