Correct confusing headers in HLSLDocs (#100017)

AvailabilityDiagnostics.rst and ExpectedDifferences.rst both had
multiple headers that were perceived to be the "top-level".

In AvailabilityDiagnostics.rst two headers had both over and underlines.
The second was the "Examples" section so it showed up in the top level
HLSL docs. The overline is removed here so it's clear it's a subheader.

In ExpectedDifferences.rst, the first header had no overline, so a few
headers that looked the same were included. The overline is added to the
top header to make clear that it's the main header.

0 files changed, 0 insertions, 0 deletions