docs: add XML documentation for security utility factories (Batch 6)

[KonradSop]

Description

Adds XML documentation to the six user-facing factory entry points under crypto/src/security. These classes are the primary discovery surface for callers who reach for an algorithm by name (JCA-style) rather than by direct type, and previously carried little or no API documentation.

• CipherUtilities — class summary describing the ALGORITHM[/MODE[/PADDING]] convention, plus <summary>/<param>/<returns>/<exception> for GetAlgorithmName and both GetCipher overloads.
• DigestUtilities — factory and one-shot helpers: all CalculateDigest, DoFinal overloads, GetAlgorithmName, GetDigest(string), GetDigest(DerObjectIdentifier), GetObjectIdentifier. Removed a stray duplicate <param name="mechanism"> block that emitted CS1571.
• MacUtilities — class summary noting required initialization, plus CalculateMac, both DoFinal overloads, GetAlgorithmName, GetMac(string), GetMac(DerObjectIdentifier).
• SignerUtilities — class summary covering the DIGESTwithCIPHER convention, plus Algorithms, GetDefaultX509Parameters (both), GetEncodingName, GetSigner (both), InitSigner (both, with full parameter docs).
• GeneratorUtilities — GetKeyGenerator (both), GetKeyPairGenerator (both).
• ParameterUtilities — GetCanonicalAlgorithmName, all four CreateKeyParameter overloads, both GetCipherParameters, both GenerateParameters, plus the ParametersWithRandom / ParametersWithContext envelope helpers (GetContext, GetRandom, IgnoreRandom, WithContext, WithRandom).

Key Accomplishments

• Discoverability: Class-level summaries explain the JCA-style naming convention each factory accepts, so IDE tooltips give callers a complete mental model from one hover.
• Accurate exception contracts: <exception> tags only added where the method body actually throws — no speculative annotations.
• Implementation comments preserved: All pre-existing // TODO[api], // TODO, and inline implementation notes left verbatim.
• Consistent style: 120-char line limit applied across all six files; <see cref="..."/> used for type/member references rather than backticks.

Verification

• Build Status: dotnet build crypto/src/BouncyCastle.Crypto.csproj -c Release — succeeded on net6.0, netstandard2.0, net461 with 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.