Currently, the output of `javadoc --help` does not cover "Command-Line Argument Files" (a.k.a. argfiles or @files) feature:

$ javadoc -help | egrep '\@|file|option|argument'
    javadoc [options] [packagenames] [sourcefiles] [@files]
where options include:
                  Override location of platform class files used for non-modular
                  Specify where to find user class files
                  Specify where to find doclet class files
                  Source file encoding name
                  Display command line options and exit
                  Print a synopsis of nonstandard options and exit
                  Specify where to find input source files for multiple modules
                  Specify where to find source files
    --add-stylesheet <file>
                  Additional stylesheet file for the generated documentation
                  Allow JavaScript in options and comments
    -author Include @author paragraphs
                  Destination directory for output files
    -docfilessubdirs
                  Recursively copy doc-file subdirectories
    -excludedocfilessubdir <name>:..
                  Exclude any doc-files subdirectories with given name
    -helpfile <file>
                  Include file that help link links to
    -html5 Generate HTML 5 output. This option is no longer required.
    --main-stylesheet <file>, -stylesheetfile <file>
                  Do not include @deprecated information
    -nosince Do not include @since information
    -overview <file>
                  Read overview documentation from HTML file
    -serialwarn Generate warning about @serial tag
    -splitindex Split index into one file per letter
                  Specify single argument custom tags
    -version Include @version paragraphs
GNU-style options may use = instead of whitespace to separate the name of an
option from its value.

Although javac has a similar feature, it provides some info:

$ javac -help
Usage: javac <options> <source files>
where possible options include:
  @<filename> Read options and filenames from file
...

Related info
========
https://docs.oracle.com/en/java/javase/14/javadoc/javadoc-command.html#GUID-EFE927BC-DB00-4876-808C-ED23E1AAEF7D