Add description for interface.

Change `getStringValue()` to `getValue()` (and also for `setValue()`).

Author: JakeQZ

src/Property/Selector/SelectorComponent.php

@@ -4,17 +4,37 @@
 
 namespace Sabberworm\CSS\Property\Selector;
 
+/**
+ * @see https://developer.mozilla.org/en-US/docs/Web/CSS/Guides/Selectors/Selector_structure
+ *
+ * A complex selector is a sequence of one or more simple and/or compound selectors that are separated by combinators,
+ * including the white space descendant combinator.
+ *
+ * This is essentially a 'selector', as a 'selector list' is a list of comma-separated selectors.
+ *
+ * A compound selector is a superset that includes a simple selector:
+ * - `li` is a simple selector (but can be treated as a compound selector);
+ * - `li:last-child` is a compound selector;
+ * - `ul li:last-child` is a complex selector as it has a combinator (the whitespace descendent combinator).
+ *
+ * A (complex) selector can be decomposed thus:
+ * selector = compound-selector [combinator, compound-selector]*
+ *
+ * It's a list of alternating types.
+ *
+ * This interface covers both types, so they can be put into an array of the interface type.
+ */
 interface SelectorComponent
 {
     /**
      * @return non-empty-string
      */
-    public function getStringValue(): string;
+    public function getValue(): string;
 
     /**
      * @param non-empty-string $value
      */
-    public function setStringValue(string $value): void;
+    public function setValue(string $value): void;
 
     /**
      * @return int<0, max>