docs(forms): add section under Validators.pattern detailing use of global and sticky flags gotcha (#39055)

Due to how the global and sticky flag make RegExp objects stateful,
adds section detailing how it is not recommended
to use these flags for control validations.

PR Close #39055

packages/forms/src/validators.ts

@@ -366,6 +366,25 @@ export class Validators {
    * <input pattern="[a-zA-Z ]*">
    * ```
    *
+   * ### Pattern matching with the global or sticky flag
+   *
+   * `RegExp` objects created with the `g` or `y` flags that are passed into `Validators.pattern`
+   * can produce different results on the same input when validations are run consecutively. This is
+   * due to how the behavior of `RegExp.prototype.test` is
+   * specified in [ECMA-262](https://tc39.es/ecma262/#sec-regexpbuiltinexec)
+   * (`RegExp` preserves the index of the last match when the global or sticky flag is used).
+   * Due to this behavior, it is recommended that when using
+   * `Validators.pattern` you **do not** pass in a `RegExp` object with either the global or sticky
+   * flag enabled.
+   *
+   * ```typescript
+   * // Not recommended (since the `g` flag is used)
+   * const controlOne = new FormControl('1', Validators.pattern(/foo/g));
+   *
+   * // Good
+   * const controlTwo = new FormControl('1', Validators.pattern(/foo/));
+   * ```
+   *
    * @param pattern A regular expression to be used as is to test the values, or a string.
    * If a string is passed, the `^` character is prepended and the `$` character is
    * appended to the provided string (if not already present), and the resulting regular