[BUG] failed to build

It appears building openapi-generator-cli.jar doesn't work properly without JAVA_HOME anymore.
Maybe JAVA_HOME should be mentioned in the instruction 1.4 - Build Projects.

✔ ~/git/github/OpenAPITools/openapi-generator [master|✔]
11:56 $ git log -1
commit 41afcd0f8646f72a5d9975cd9abbb7c972124c16 (HEAD -> master, origin/master, origin/HEAD)
Author: Minjae Kim <haedaal@gmail.com>
Date:   Fri Jun 18 17:34:38 2021 +0900

    Handle boolean enum correctly in typescript-axios (#9025)
    
    can't use enum because of TS18033
12:00 $ mvn clean install
..
[INFO] --- maven-javadoc-plugin:3.1.0:jar (attach-javadocs) @ openapi-generator-core ---
[INFO] ------------------------------------------------------------------------
[INFO] Reactor Summary for openapi-generator-project 5.2.0-SNAPSHOT:
[INFO] 
[INFO] openapi-generator-project .......................... SUCCESS [  1.537 s]
[INFO] openapi-generator-core ............................. FAILURE [  4.290 s]
[INFO] openapi-generator (core library) ................... SKIPPED
[INFO] openapi-generator (executable) ..................... SKIPPED
[INFO] openapi-generator (maven-plugin) ................... SKIPPED
[INFO] openapi-generator-gradle-plugin (maven wrapper) .... SKIPPED
[INFO] openapi-generator-online ........................... SKIPPED
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  6.131 s
[INFO] Finished at: 2021-06-18T12:00:53+02:00
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-javadoc-plugin:3.1.0:jar (attach-javadocs) on project openapi-generator-core: MavenReportException: Error while generating Javadoc: Unable to find javadoc command: The environment variable JAVA_HOME is not correctly set. -> [Help 1]
[ERROR] 
[ERROR] To see the full stack trace of the errors, re-run Maven with the -e switch.
[ERROR] Re-run Maven using the -X switch to enable full debug logging.
[ERROR] 
[ERROR] For more information about the errors and possible solutions, please read the following articles:
[ERROR] [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/MojoExecutionException
[ERROR] 
[ERROR] After correcting the problems, you can resume the build with the command
[ERROR]   mvn <args> -rf :openapi-generator-core

Setting JAVA_HOME solved the issue: JAVA_HOME=/usr/lib/jvm/java-11-openjdk-amd64/ mvn clean install

Start with the linked “1.4 - Build Projects” instructions and compare them with the reported mvn clean install failure. Update the build guidance to cover the JAVA_HOME prerequisite, then verify the documented command against the reported Java setup.