[selectors] Restore definition of the subject of a selector, since that is a term that has been around to describe how selectors work since 1998 at least, and shouldn't just disappear from our specs.

Author: fantasai

selectors/Overview.bs

@@ -501,6 +501,17 @@ Selector Syntax and Structure</h2>
 <h3 id="structure">
 Structure and Terminology</h3>
 
+	The elements of a document tree that are represented by a <a>selector</a>
+	are the <dfn title="subject">subjects</dfn> of the selector.
+	The subjects of a selector are the elements represented
+	by the last <a>compound selector</a> in the selector.
+	Thus a selector consisting of a single <a>compound selector</a>
+	represents any element satisfying its requirements.
+	Prepending another <a>compound selector</a> and a <a>combinator</a>
+	to a sequence imposes additional matching constraints,
+	so the <a>subjects</a> of the selector are always a subset of
+	the elements represented by the last <a>compound selector</a>.
+
 	The term <dfn export>selector</dfn> can refer to a <a>simple selector</a>,
 	<a>compound selector</a>, <a>complex selector</a>, or <a>selector list</a>.