Add docs for StringConcatToTextBlock

PiperOrigin-RevId: 698226007

docs/bugpattern/StringConcatToTextBlock.md

@@ -0,0 +1,30 @@
+Using [text blocks] for strings that span multiple lines can make code easier to
+read.
+
+For example, prefer this:
+
+```java
+String message =
+    """
+    'The time has come,' the Walrus said,
+    'To talk of many things:
+    Of shoes -- and ships -- and sealing-wax --
+    Of cabbages -- and kings --
+    And why the sea is boiling hot --
+    And whether pigs have wings.'
+    """;
+```
+
+instead of this:
+
+```java
+String message =
+    "'The time has come,' the Walrus said,\n"
+        + "'To talk of many things:\n"
+        + "Of shoes -- and ships -- and sealing-wax --\n"
+        + "Of cabbages -- and kings --\n"
+        + "And why the sea is boiling hot --\n"
+        + "And whether pigs have wings.'\n";
+```
+
+[text blocks]: https://docs.oracle.com/en/java/javase/23/text-blocks/index.html