Clarify further what goes into

Author: davepeck

Doc/library/string.templatelib.rst

@@ -259,8 +259,9 @@ Types
       :type: str
 
       For interpolations created by t-string literals, :attr:`!expression`
-      is the source text found inside the curly braces (``{`` & ``}``),
-      including any whitespace, but excluding the curly braces themselves.
+      is the expression text found inside the curly braces (``{`` & ``}``),
+      including any whitespace, excluding the curly braces themselves,
+      and ending before the first ``!``, ``:``, or ``=`` if any is present.
       For manually created interpolations, :attr:`!expression` is the arbitrary
       string provided when constructing the interpolation instance.