[xkey doc] Add example for use with space separated headers

[andrerom]

Adjusts docs for two things:

• That space separated values also work (like in the original Surrogate-Key case)
• Reflect that purge can take several values now (since 0.10.2 in f723b4c25f292fcad85c9fac88e20d64e2d08ea0)

[dridi]

Hello @andrerom, thanks for the pull request. Could you please remove the _ markers and let it render as regular documentation? Also, once it's done, please squash your pull request into a single commit.

[andrerom]

@Dridi fixed and squashed

[dridi]

@andrerom thanks! @daghf back to you ;)

[daghf]

Thanks. There are still a couple of typos, but that can be addressed. I'll merge this now and follow up with a few fixes.