docs: add XML documentation for CMS SignerInfoGenerator (Batch 16b)

[KonradSop]

Description

Adds XML documentation to SignerInfoGenerator and SignerInfoGeneratorBuilder - the
pre-configured signer types used with CmsSignedDataGenerator.AddSignerInfoGenerator (Batch 12a-3,
#686):

• SignerInfoGenerator - class summary and write-side workflow; properties (Certificate,
  GeneratedVersion, SignatureFactory, attribute generators, SignerID); NewBuilder.
• SignerInfoGeneratorBuilder - <remarks> for direct-signature vs default signed attributes;
  SetDirectSignature, WithSignedAttributeGenerator, WithUnsignedAttributeGenerator, and both
  Build overloads (X.509 certificate vs subject key identifier).

Legacy /** block comments converted to XML.

Key Accomplishments

• CMS write-side discoverability: IDE tooltips now cover the advanced signer configuration path
  alongside the simpler AddSigner overloads on CmsSignedDataGenerator.
• Accurate defaults: documents that a null signed-attribute generator selects
  DefaultSignedAttributeTableGenerator unless direct signature is enabled.
• Consistent style: matches Batch 14-15 documentation conventions (workflow + <see cref> +
  <remarks> where helpful).
• No overlap with merged work: single file, disjoint from CmsSignedDataGenerator (docs: add XML documentation for CMS SignedData generator (Batch 12a-3) #686) and
  signer read-side types (docs: add XML documentation for CMS signer types (Batch 14b) #699).

Verification

• Build Status: dotnet build crypto/src/BouncyCastle.Crypto.csproj -c Release - 0 errors, no new warnings.
• Line length: all new or modified /// lines are at most 120 characters.
• 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.