docs: add XML documentation for CMS signer types (Batch 14b)

[KonradSop]

Description

Adds XML documentation to the CMS signer-information API used to identify signers and verify
CMS SignedData signatures:

• SignerInformation — class summary and verify workflow, properties (digest/signature
  algorithms, attributes, counter signatures), GetSignature, GetContentDigest,
  GetCounterSignatures, Verify overloads, and static attribute helpers.
• SignerID — issuer/serial number and subject key identifier matching; equality semantics.
• OriginatorID — originator matching by issuer/serial number or subject key identifier.

Legacy /** block comments in SignerInformation converted to XML.

Key Accomplishments

• CMS verify discoverability: IDE tooltips now explain how to match a signer and verify
  signatures with a public key or certificate.
• Accurate exception contracts: documented only where the implementation throws.
• Focused scope: three CMS signer 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.
• 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.