also adds/improves doc comments on related functions, and adds tests of
comparisons RSA keys and ECDSA keys. these tests failed as expected
before the function was changed, e.g.:
```text
Executing tests from //pkg/util/pki:go_default_test
---------------------------------------------------
--- FAIL: TestPublicKeysEqualECDSA (0.00s)
generate_test.go:492: got an incorrect match from different curves:
pub1 type: "P-256"
pub2 type: "P-521"
--- FAIL: TestPublicKeysEqualRSA (0.00s)
generate_test.go:560: got an incorrect match from different RSA keys:
pub1: &rsa.PublicKey{N:2293...<snip>...8869, E:65537}
pub2: &rsa.PublicKey{N:2293...<snip>...8869, E:3}
```
Signed-off-by: Ashley Davis <ashley.davis@jetstack.io>
3df1173a22