Improve documentation for WordPress.DB.SlowDBQuery

- Rewrite standard text to mention all four flagged keys, clarify
  that both array assignments and query-parameter strings are
  detected.
- Replace code examples with examples that stay within 48-character
  limit and use `<em>` tags.
- Remove second code comparison that incorrectly showed
  `tax_query` and `meta_key` as valid when the sniff warns on all
  four keys.

Author: rodrigoprimo

WordPress/Docs/DB/SlowDBQueryStandard.xml

@@ -5,78 +5,28 @@
     >
     <standard>
     <![CDATA[
-    For performance reason, it's recommended to avoid doing lots of meta queries.
-    The underlying SQL request generated by those queries can be extremely slow queries especially for complexe meta query.
+    Usage of the meta_query, meta_key, meta_value, and tax_query keys in array assignments and query-parameter strings is discouraged and should be carefully evaluated. When passed to WordPress database query APIs, these keys can lead to slow queries, especially on large datasets.
     ]]>
     </standard>
     <code_comparison>
-        <code title="Valid: Using tax_query to retrieve a list of posts">
+        <code title="Valid: No potentially slow query keys.">
         <![CDATA[
-$query = new WP_Query(
-    array(
-        'tax_query' => array(
-            array(
-                'taxonomy' => 'color',
-                'field'    => 'slug',
-                'terms'    => array(
-                    'blue',
-                    'red',
-                )
-            )
-        )
-    )
+$args = array(
+    'post_type' => 'post',
 );
-        ]]>
-        </code>
-        <code title="Invalid: Using meta_query to retrieve a list of posts">
-        <![CDATA[
-$query = new WP_Query(
-    array(
-        'meta_query' => array(
-            array(
-                'key'     => 'color',
-                'compare' => 'IN',
-                'value'   => array(
-                    'blue',
-                    'red',
-                )
-            )
-        )
-    )
-);
-        ]]>
-        </code>
-    </code_comparison>
-    <code_comparison>
-        <code title="Valid: in case of binary metadata (true/false) check if meta_key exist and remove it otherwise">
-        <![CDATA[
-// mark post 123 as "featured"
-update_post_meta( 123, 'is_featured', true );
 
-// remove "featured" state for post 456
-delete_post_meta( 456, 'is_featured' );
 
-$query = new WP_Query(
-    array(
-        'meta_key' => 'is_featured'
-    ),
-);
+$query = 'post_type=post&orderby=date';
         ]]>
         </code>
-        <code title="Invalid: check for the value of a binary metadata, which is slower">
+        <code title="Invalid: Using potentially slow query keys.">
         <![CDATA[
-// mark post 123 as "featured"
-update_post_meta( 123, 'is_featured', true );
-
-// mark post 456 as not "featured"
-update_post_meta( 456, 'is_featured', false );
-
-$query = new WP_Query(
-    array(
-        'meta_key' => 'is_featured',
-        'meta_value' => true
-    ),
+$args = array(
+    '<em>meta_key</em>'   => 'color',
+    '<em>meta_value</em>' => 'blue',
 );
+
+$query = 'post_type=post&<em>meta_key</em>=featured';
         ]]>
         </code>
     </code_comparison>