Merge pull request WebAssembly#481 from takikawa/fix-constexpr-note

[spec] Update global.get note on constant expressions

Author: rossberg

document/core/valid/instructions.rst

@@ -2544,7 +2544,7 @@ Constant Expressions
 
 
 .. note::
-   Currently, constant expressions occurring in :ref:`globals <syntax-global>`, :ref:`element <syntax-elem>`, or :ref:`data <syntax-data>` segments are further constrained in that contained |GLOBALGET| instructions are only allowed to refer to *imported* globals.
+   Currently, constant expressions occurring in :ref:`globals <syntax-global>` are further constrained in that contained |GLOBALGET| instructions are only allowed to refer to *imported* or *previously defined* globals. Constant expressions occurring in :ref:`tables <syntax-table>` may only have |GLOBALGET| instructions that refer to *imported* globals.
    This is enforced in the :ref:`validation rule for modules <valid-module>` by constraining the context :math:`C` accordingly.
 
    The definition of constant expression may be extended in future versions of WebAssembly.