[ObjC] Typos and comment improvements.

objectivec/GPBDescriptor.m

@@ -508,7 +508,8 @@ - (instancetype)initWithFieldDescription:(void *)description
     BOOL isMessage = GPBDataTypeIsMessage(dataType);
     BOOL isMapOrArray = GPBFieldIsMapOrArray(self);
 
-    // If proto3 optionals weren't know, compute the flag for the rest of the runtime.
+    // If proto3 optionals weren't known (i.e. generated code from an
+    // older version), compute the flag for the rest of the runtime.
     if (!proto3OptionalKnown) {
       // If it was...
       //  - proto3 syntax
@@ -530,7 +531,7 @@ - (instancetype)initWithFieldDescription:(void *)description
       // autocreation.
       hasOrCountSel_ = SelFromStrings(NULL, coreDesc->name, "_Count", NO);
     } else {
-      // Know it is a single field; it gets has/setHas selectors if...
+      // It is a single field; it gets has/setHas selectors if...
       //  - not in a oneof (negative has index)
       //  - not clearing on zero
       if ((coreDesc->hasIndex >= 0) &&

objectivec/GPBDescriptor_PackagePrivate.h

@@ -155,9 +155,9 @@ typedef NS_OPTIONS(uint32_t, GPBDescriptorInitializationFlags) {
   // breaking change in the runtime.
   GPBDescriptorInitializationFlag_UsesClassRefs     = 1 << 2,
 
-  // This flag is used to indicate that the generated sources already containg
+  // This flag is used to indicate that the generated sources already contain
   // the `GPBFieldClearHasIvarOnZero` flag and it doesn't have to be computed
-  // at startup, this allows older generated code to still work with the
+  // at startup. This allows older generated code to still work with the
   // current runtime library.
   GPBDescriptorInitializationFlag_Proto3OptionalKnown = 1 << 3,
 };

objectivec/GPBUtilities.h

@@ -95,7 +95,7 @@ BOOL GPBMessageHasFieldSet(GPBMessage *self, GPBFieldDescriptor *field);
 void GPBClearMessageField(GPBMessage *self, GPBFieldDescriptor *field);
 
 /**
- * Clears the given onoof field for the given message.
+ * Clears the given oneof field for the given message.
  *
  * @param self  The message for which to clear the field.
  * @param oneof The oneof to clear.