[keyvault] fix doc comments

[gracewilcox]

When we changed some of the names, not all the doc comments were changed to match the new names. This pull request that issue and updates the doc comments

[chlowell]

What do you think of rewriting the comments to remove the redundant type name e.g. "// Elliptic curve name. For valid values, see CurveName."? I'm thinking, people reading the comment have the type or its name in front of them, and in the rendered docs type names link to definitions