SecurityPkg/FvReportPei: Improve CheckStoredHashFv() description

Adds more detail to the function description so the dependencies
and outcomes of the verification process are easier for a reviewer
and consumer to understand.

Signed-off-by: Michael Kubacki <michael.kubacki@microsoft.com>

SecurityPkg/FvReportPei/FvReportPei.c

@@ -343,17 +343,38 @@ GetHashInfo (
 }
 
 /**
-  Verify and report pre-hashed FVs.
+ Verifies and reports pre-hashed Firmware Volumes (FVs).
 
-  Doing this must be at post-memory to make sure there's enough memory to hold
-  all FVs to be verified. This is necessary for mitigating TOCTOU issue.
+  This function should be called after memory initialization (post-memory phase)
+  to ensure sufficient memory is available to hold all FVs for verification.
 
-  This function will never return if the verification is failed.
+  Verifying FVs at this stage is critical to mitigate Time-of-Check to Time-of-Use (TOCTOU) vulnerabilities.
+
+  FV Verification Step:
+
+  This function relies upon an instance of gEdkiiPeiFirmwareVolumeInfoStoredHashFvPpiGuid being installed. If
+  an instance of the PPI is not found, EFI_NOT_FOUND is returned.
+
+  If the gEdkiiPeiFirmwareVolumeInfoStoredHashFvPpiGuid PPI instance does not have valid hash information for the
+  current boot mode (hash info is NULL), verification is treated as successful.
+
+  FV Reporting Step:
+
+  If FV verification is successful, the function reports the FVs to PEI and/or DXE core for further processing by:
+
+  1. Installing a FV HOB.
+  2. Installing a FV Info PPI.
+  3. Reporting a status code with the value from PcdStatusCodeFvVerificationPass to indicate that FV verification
+     passed.
+
+  If FV verification fails, the function reports a status code with the value from PcdStatusCodeFvVerificationFail
+  and returns a failure status code.
 
   @param[in] PeiServices      General purpose services available to every PEIM.
   @param[in] BootMode         Current boot mode.
 
   @retval EFI_SUCCESS         The function completed successfully.
+  @retval EFI_NOT_FOUND       No valid gEdkiiPeiFirmwareVolumeInfoStoredHashFvPpiGuid PPI instance found.
 **/
 STATIC
 EFI_STATUS