Tweak CSR parsing errors/documentation

[djc]

Replacement for

• Revise errors for CertificateSigningRequestParams::from_der #388

[cpu]

One wording nit, but in sum LGTM. Thanks!

[dwhjames]

Following up from:

• Revise errors for CertificateSigningRequestParams::from_der #388

Note, there are now three distinct ways that the context of a CSR is indicated in an error name:

rcgen/rcgen/src/error.rs

Lines 9 to 10 in 39e8a81

	/// The given certificate signing request couldn't be parsed
	CouldNotParseCertificationRequest,

rcgen/rcgen/src/error.rs

Lines 13 to 15 in 39e8a81

	/// The CSR signature is invalid
	#[cfg(feature = "x509-parser")]
	InvalidRequestSignature,

rcgen/rcgen/src/error.rs

Lines 42 to 43 in 39e8a81

	/// Unsupported field when generating a CSR
	UnsupportedInCsr,

Probably other things to critique about Error::InvalidSignatureInCsr, but it does avoid creating a 3rd naming style.

[djc]

That's fair. I've renamed InvalidRequestSignature to InvalidCertificationRequestSignature to alleviate that concern.