It looks like the docs were recently updated to reflect the History Server's support for incomplete applications, but they still had wording that suggested only completed applications were viewable.  This fixes that.

My editor also introduced several whitespace removal changes, that I hope are OK, as text files shouldn't have trailing whitespace.  To verify they're purely whitespace changes, add `&w=1` to your browser address.  If this isn't acceptable, let me know and I'll update the PR.

I also didn't think this required a JIRA.  Let me know if I should create one.

Not tested

Author: Michael Gummelt <mgummelt@mesosphere.io>

Closes #12045 from mgummelt/update-history-docs.