Add clarification of logging.file (fixes gh-1416)

Author: Dave Syer

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

@@ -720,7 +720,7 @@ If your terminal supports ANSI, color output will be used to aid readability.
 By default, log files are written to `spring.log` in your `temp` directory and rotate at
 10 Mb. You can easily customize the output folder by setting the `logging.path` property
 (for example in your `application.properties`). It is also possible to change the filename
-using a `logging.file` property.
+using a `logging.file` property. Note that if `logging.file` is used, then setting `logging.path` has no effect.
 
 As with console output, `ERROR`, `WARN` and `INFO` level messages are logged by default.