Add a warning to the statistics client about caching

[hnrkndrssn]

I had to add the warning to the Statistics property as well on the *RepositoriesClient otherwise the warning won't show up in intellisense when you do github.Repositories.Statistics

Fixes #1814

[ryangribble]

Unless I'm totally wrong on this, I thought the <remarks> tag didnt appear in intellisense - and it was only <summary> that does?

I'm also wondering whether we should embed the full text from GitHub here (what if the text changes in the future?) or whether we could just have a simpler note like "Note that the GitHub API uses caching on these endpoints, see https://developer.github.com/v3/repos/statistics/#a-word-about-caching for futther details"

This has now been sorted

[shiftkey]

release_notes: Add comment about caching when calling the repository statistics endpoint