doc/arch.md: say when numeric parameters are checked

The AST doesn't do detailed checking of numeric parameters.
That's deferred to the separate validation step.

Change-Id: I85aada7292fcc9757f9fd54e2d9a50ad1405ef9c
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/29940
Reviewed-by: dan sinclair <dsinclair@chromium.org>
Commit-Queue: dan sinclair <dsinclair@chromium.org>

docs/arch.md

@@ -54,10 +54,21 @@ grammar in the naming of the parse methods.
 
 ## AST
 
-After generation the AST will run an `IsValid` method which verifies that the
+The Abstract Syntax Tree is a directed acyclic graph of nodes encoding
-produced AST is structurally correct. This means that things like `if`
+the structure of the WGSL program.
-statements have a condition and body attached.
 
+Many AST nodes have numeric parameters. For example, the ast::StrideDecoration
+node has numeric stride parameter, which is a count of the number of bytes from
+the start of one array element to the start of the next.  The AST node itself
+does not constrain the set of stride values that you can set, aside from storing
+it as an unsigned integer.
+
+After creating the AST for the whole WGSL program, Tint will run an `IsValid`
+method which verifies that the produced AST is structurally correct.  This
+means that things like `if` statements have a condition and body attached.
+
+A later phase of processing, Validation, will check properties such as whether
+array strides satisfy all the requirements in the WGSL specification.
 
 ## Type Determiner