Docs: Clean up HCE dev guide

Fixed various typos and minor errors in the HCE dev guide. Also added a
note to clarify when CATEGORY_PAYMENT should be used vs CATEGORY_OTHER.

Issue: 11991294

Change-Id: Ifa3f30fe5f35715a4cea3f3565b6f3771f44814a
Signed-off-by: Trevor Johns <trevorjohns@google.com>
1 file changed