Update docs regarding regex in ci rules

doc/ci/variables/README.md

@@ -571,9 +571,12 @@ Below you can find supported syntax reference:
    - `$VARIABLE =~ /^content.*/`
    - `$VARIABLE_1 !~ /^content.*/` (introduced in GitLab 11.11)
 
-   It is possible perform pattern matching against a variable and regular
-   expression. Expression like this evaluates to truth if matches are found
-   when using `=~`. It evaluates to truth if matches are not found when `!~` is used.
+   Variable pattern matching with regular expressions uses the
+   [RE2 regular expression syntax](https://github.com/google/re2/wiki/Syntax).
+   Expressions evaluate as `true` if:
+
+   - Matches are found when using `=~`.
+   - Matches are *not* found when using `!~`.
 
    Pattern matching is case-sensitive by default. Use `i` flag modifier, like
    `/pattern/i` to make a pattern case-insensitive.

doc/ci/yaml/README.md

@@ -857,7 +857,10 @@ In this example, if the first rule:
 
 `rules:if` differs slightly from `only:variables` by accepting only a single
 expression string, rather than an array of them. Any set of expressions to be
-evaluated should be conjoined into a single expression using `&&` or `||`. For example:
+evaluated should be conjoined into a single expression using `&&` or `||`, and use
+the [variable matching syntax](../variables/README.md#supported-syntax).
+
+For example:
 
 ```yaml
 job: