[syntax] p2 Add "X-expression" in the list

All grammars defined in [expr] that have the form X-expression are intended to interpret as expressions. And we use the literal word "expression" in another subclause such as [basic.def.odr] to define some rules for these expressions.

extern int b;
int a = b;

b as the grammar component initializer that is an assignment-expression should eventually obey the rule defined for "expression" in [basic.def.odr](i.e. the variable b is odr-used by an expression). In the current draft, we just have a note specifying that X-expression is the syntax of an expression
[expr.pre] p1

[Note 1: [expr] defines the syntax, order of evaluation, and meaning of expressions. An expression is a sequence of operators and operands that specifies a computation. An expression can result in a value and can cause side effects. — end note]

Augment [syntax] p2 to cover the meaning of the note

Names for syntactic categories have generally been chosen according to the following rules:

• [...]
• X-expression is an expression with no context-dependent meaning.

Start by reading [syntax] p2 and compare its category-naming rules with the note in [expr.pre] p1. Update the wording in [syntax] p2 so the stated meaning of X-expression is covered, then review the surrounding standard text for consistency.