Add more information for CherrypickOptions

Author: kshyatt

base/libgit2/types.jl

@@ -527,6 +527,15 @@ end
     LibGit2.CherrypickOptions
 
 Matches the [`git_cherrypick_options`](https://libgit2.github.com/libgit2/#HEAD/type/git_cherrypick_options) struct.
+
+The fields represent:
+  * `version`: version of the struct in use, in case this changes later. For now, always `1`.
+  * `mainline`: if cherrypicking a merge commit, specifies the parent number (starting at `1`)
+    which will allow cherrypick to apply the changes relative to that parent. Only relevant if
+    cherrypicking a merge commit. Default is `0`.
+  * `merge_opts`: options for merging the changes in. See [`MergeOptions`](@ref) for more information.
+  * `checkout_opts`: options for the checkout of the commit being cherrypicked. See [`CheckoutOptions`](@ref)
+     for more information.
 """
 @kwdef struct CherrypickOptions
     version::Cuint = 1