[xmlsec] Documentation errata

[Jesse Pelton]

I've noticed a few minor errors in the documentation:

- The description of xmlSecSimpleKeysMngrLoadPemKey's keyPwdCallback
parameter has "cllback" where "callback" is intended.
- The description of xmlSecDSigGenerate()'s signNode and result parameters
refer to validation (of signatures) rather than generation.
- The description of xmlSecDSigResult's buffer member has
"cannonicalization" where "canonicalization" is intended.
- The description of xmlSecReferenceResult's self member has
"<dsig:Refernece>" where "<dsig:Reference>" is intended.
- The description of xmlSecReferenceResult's buffer member seems incomplete.
Consider "the pointer to digested content (valid only if the storeReferences
flag and/or storeManifests flag in xmlSecDSigCtx is set)."
- The xmlSecSignatureCreate description has "mantadory" where "mandatory" is
intended.

Note that this is just a list of what I stumbled over in the course of using
the documentation, not the result of an exhaustive review.