docs: add XML documentation for CMS CompressedData stream generator (Batch 13c-1)

[KonradSop]

Description

Adds XML documentation to CmsCompressedDataStreamGenerator — the streaming generator for CMS CompressedData messages:

• Class summary — describes the Open/write/close workflow and a usage example; notes that only ZLIB compression is supported.
• Stream-handling note — closing the returned stream finalizes the CMS structure but does not close the underlying stream; callers close it separately.
• ZLib — documents the only supported compression OID.
• Constructor and SetBufferSize.
• Open overloads — all three documented; <exception> on the core overload where it rejects non-ZLIB algorithms.

Legacy /** block comments converted to XML.

Key Accomplishments

• CMS compression discoverability: IDE tooltips now cover the full public generator surface (0 → 7 <summary> blocks).
• Accurate exception contract: ArgumentException documented only on the Open overload that validates the compression OID.
• Consistent style: matches Batch 12a/13a documentation conventions.

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.