Unify console and file logback patterns

Update the file log pattern to write the thread name after the `---`
separator. The allows both file and console logs to be parsed in the
same way.

Fixes gh-2136

spring-boot-docs/src/main/asciidoc/spring-boot-features.adoc

@@ -734,6 +734,7 @@ The following items are output:
 * Log Level -- `ERROR`, `WARN`, `INFO`, `DEBUG` or `TRACE`.
 * Process ID.
 * A `---` separator to distinguish the start of actual log messages.
+* Thread name -- Enclosed in square brackets (may be truncated for console output).
 * Logger name -- This is usually the source class name (often abbreviated).
 * The log message.