Document that the default rolling policy for Log4j2 requires logging.file.path to be set

[snicoll]

The current configuration is as follows

spring-boot/spring-boot-project/spring-boot/src/main/resources/org/springframework/boot/logging/log4j2/log4j2-file.xml

Lines 24 to 39 in 6cbe9a4

	<RollingFile name="File" fileName="${sys:LOG_FILE}" filePattern="${sys:LOG_PATH}/$${date:yyyy-MM}/app-%d{yyyy-MM-dd-HH}-%i.log.gz">
	<Select>
	<SystemPropertyArbiter propertyName="FILE_LOG_STRUCTURED_FORMAT">
	<StructuredLogLayout format="${sys:FILE_LOG_STRUCTURED_FORMAT}" charset="${sys:FILE_LOG_CHARSET}"/>
	</SystemPropertyArbiter>
	<DefaultArbiter>
	<PatternLayout pattern="${sys:FILE_LOG_PATTERN}" charset="${sys:FILE_LOG_CHARSET}"/>
	</DefaultArbiter>
	</Select>
	<Filters>
	<ThresholdFilter level="${sys:FILE_LOG_THRESHOLD:-TRACE}"/>
	</Filters>
	<Policies>
	<SizeBasedTriggeringPolicy size="10 MB"/>
	</Policies>
	</RollingFile>

Rolling a file called myapp.log leads to a strange folder structure that expects LOG_PATH to be set, something like:

.
├── ${sys:LOG_PATH}
│   └── 2025-12
│       ├── app-2025-12-15-09-1.log.gz
│       ├── app-2025-12-15-09-2.log.gz
│       ├── app-2025-12-15-09-3.log.gz
│       ├── app-2025-12-15-09-4.log.gz
│       └── app-2025-12-15-09-5.log.gz

[snicoll]

We've decided to document the requirement as making LOG_PATH available may have a cascading effect on other settings. We'll fix the issue proper in the next feature release, see #47260