PEL: Document SRC hex word 2 fields in the code

The following hex word 2 fields are always left at zero
when creating OpenBMC PELs:
* Platform dump status (doesn't apply to OpenBMC errors)
* Partition boot type (doesn't applly to OpenBMC errors)
* Platform boot mode
  * Boot side is always Temporary = 0
  * Boot speed is always Fast = 0

Signed-off-by: Matt Spinler <spinler@us.ibm.com>
Change-Id: I88a0bf9176fbfb8b62eafce73d74f49c9a8b0a66
diff --git a/extensions/openpower-pels/src.cpp b/extensions/openpower-pels/src.cpp
index cdafce3..b96cb8f 100644
--- a/extensions/openpower-pels/src.cpp
+++ b/extensions/openpower-pels/src.cpp
@@ -109,14 +109,21 @@
     // There are multiple fields encoded in the hex data words.
     std::for_each(_hexData.begin(), _hexData.end(),
                   [](auto& word) { word = 0; });
+
+    // Hex Word 2 Nibbles:
+    //   MIGVEPFF
+    //   M: Partition dump status = 0
+    //   I: System boot state = TODO
+    //   G: Partition Boot type = 0
+    //   V: BMC dump status = TODO
+    //   E: Platform boot mode = 0 (side = temporary, speed = fast)
+    //   P: Platform dump status = TODO
+    //  FF: SRC format, set below
+
     setBMCFormat();
     setBMCPosition();
     setMotherboardCCIN(dataIface);
 
-    // Partition dump status and partition boot type always 0 for BMC errors.
-    //
-    // TODO: Fill in other fields that aren't available yet.
-
     // Fill in the last 4 words from the AdditionalData property contents.
     setUserDefinedHexWords(regEntry, additionalData);