[0035] Clarify Length for packed types

proposals/0035-linalg-matrix.md

@@ -695,15 +695,18 @@ uint Matrix::Length();
 
 Requires `Wave` or `ThreadGroup` scope matrix.
 
-Returns the number of matrix components accessible to the current thread. The
-mapping and distribution of threads to matrix elements is opaque and
-implementation-specific. The value returned by `Length` may be different
-for each thread. The sum of the values returned by `Length` across all
-threads must be greater than or equal to the total number of matrix elements.
-Some implementations may map multiple threads to the same matrix element.
-Therefore, developers should take this into consideration when programming
-side-effects, such as atomic operations and/or UAV writes, within user-defined
-matrix operations.
+Returns the number of matrix components accessible to the current thread. If the
+matrix's elements are stored in a packed type, `Length` will return the number of
+packed elements (e.g. if a thread has 8 accessible elements of `int8` type 
+packed into 2 `int8_t4_packed`, `Length` will return 2). The mapping and
+distribution of threads to matrix elements is opaque and
+implementation-specific. The value returned by `Length` may be different for
+each thread. The sum of the values returned by `Length` across all threads must
+be greater than or equal to the total number of matrix elements. Some
+implementations may map multiple threads to the same matrix element. Therefore,
+developers should take this into consideration when programming side-effects,
+such as atomic operations and/or UAV writes, within user-defined matrix
+operations.
 
 May be called from non-uniform control flow. However, given the above rules,
 calling `Length` from divergent threads may result in unpredictable behavior.