Editorial: Add brief explanations to examples

These examples form the only quick reference material in the spec. While MDN goes into more depth, brief explanations of what is happening in this syntax don't take up much room and may make it easier to tell what's going on, especially with regexp parts.

The example explaining pattern strings is also expanded to be explicit that regular expressions are enclosed in parentheses.

Resolves #229.

spec.bs

@@ -77,6 +77,8 @@ It can be constructed using a string for each component, or from a shorthand str
     * `http://example.com/products/`
     * `https://example.com:8443/blog/our-greatest-product-ever`
   </ul>
+
+  This is a fairly simple pattern which requires most components to either match an exact string, or allows any string ("`*`"). The [=URL pattern/pathname component=] matches any path with at least two `/`-separated path components, the first of which is captured as "`category`".
 </div>
 
 <div class="example" id="example-intro-2">
@@ -124,6 +126,10 @@ It can be constructed using a string for each component, or from a shorthand str
     * `https://nx.shop.example/products/01?speed=5#reviews`
     * `https://shop.example/products/chair#reviews`
   </ul>
+
+  This is a more complicated pattern which includes:
+  * [=part/modifier/optional=] parts marked with `?` (braces are needed to make it unambiguous exactly what is optional), and
+  * a [=part/type/regexp=] part named "`id`" which uses a regular expression to define what sorts of substrings match (the parentheses are required to mark it as a regular expression, and are not part of the regexp itself).
 </div>
 
 <div class="example" id="example-intro-3">
@@ -168,6 +174,8 @@ It can be constructed using a string for each component, or from a shorthand str
     * `https://discussion.example/forum/admin/`
     * `http://discussion.example:8080/admin/update?id=1`
   </ul>
+
+  This pattern demonstrates how pathnames are resolved relative to a base URL, in a similar way to relative URLs.
 </div>
 
 <h3 id=urlpattern-class>The {{URLPattern}} class</h3>
@@ -951,7 +959,7 @@ It can be [=parse a pattern string|parsed=] to produce a [=/part list=] which de
 <div class="example" id="example-pattern-strings">
   Pattern strings can contain capture groups, which by default match the shortest possible string, up to a component-specific separator (`/` in the pathname, `.` in the hostname). For example, the pathname pattern "`/blog/:title`" will match "`/blog/hello-world`" but not "`/blog/2012/02`".
 
-  A regular expression can also be used instead, so the pathname pattern "`/blog/:year(\\d+)/:month(\\d+)`" will match "`/blog/2012/02`".
+  A regular expression enclosed in parentheses can also be used instead, so the pathname pattern "`/blog/:year(\\d+)/:month(\\d+)`" will match "`/blog/2012/02`".
 
   A group can also be made <span class="allow-2119">optional</span>, or repeated, by using a modifier. For example, the pathname pattern "`/products/:id?"` will match both "`/products`" and "`/products/2`" (but not "`/products/`"). In the pathname specifically, groups automatically require a leading `/`; to avoid this, the group can be explicitly deliminated, as in the pathname pattern "`/products/{:id}?`".