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.

Author: pow2clk

clang/docs/HLSL/AvailabilityDiagnostics.rst

@@ -52,7 +52,6 @@ If the compilation target is a shader library, only availability based on shader
 
 As a result, availability based on specific shader stage will only be diagnosed in code that is reachable from a shader entry point or library export function. It also means that function bodies might be scanned multiple time. When that happens, care should be taken not to produce duplicated diagnostics.
 
-========
 Examples
 ========
 
@@ -62,7 +61,7 @@ For the example below, the ``WaveActiveCountBits`` API function became available
 The availability of ``ddx`` function depends on a shader stage. It is available for pixel shaders in shader model 2.1 and higher, for compute, mesh and amplification shaders in shader model 6.6 and higher. For any other shader stages it is not available.
 
 Compute shader example
-======================
+----------------------
 
 .. code-block:: c++
 
@@ -94,7 +93,7 @@ With strict diagnostic mode, in addition to the 2 errors above Clang will also e
    <>:7:13: error: 'WaveActiveCountBits' is only available on Shader Model 6.5 or newer
 
 Shader library example
-======================
+----------------------
 
 .. code-block:: c++
 

clang/docs/HLSL/ExpectedDifferences.rst

@@ -1,4 +1,4 @@
-
+===================================
 Expected Differences vs DXC and FXC
 ===================================