Wording fixes and little extra detail in one point

design-documents/bit-precise-types.rst

@@ -424,8 +424,8 @@ It has the following negatives:
   from an ABI boundary would require masking the operands.
 
 - On AArch32 this could cause surprises to developers, given that on this
-  architecture small Fundamental Data Types are have zero- or sign-extended
-  extra bits.  So a ``char`` would not have the same representation as a
+  architecture small Fundamental Data Types have zero- or sign-extended extra
+  bits.  So a ``char`` would not have the same representation as a
   ``_BitInt(8)`` on this architecture.
 
 - If used in calls to variadic functions which were written for standard
@@ -454,7 +454,9 @@ It has the following negatives:
   boundary.
 
 - On AArch64 this would not match the expectation of developers, with
-  ``_BitInt(8)`` not matching the representation of a ``char``.
+  ``_BitInt(8)`` not matching the representation of a ``char``.  That said, a
+  ``_BitInt(8)`` at a PCS boundary would be a valid ``char`` (since the ``char``
+  bits are unspecified and the ``_BitInt(8)`` bits are specified).
 
 Summary, suggestion, and reasoning
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