Remove misleading CLI documentation for boolean options

[ricksbrown]

Most of the on/off cli flags are backed by BooleanOptionHandler which
does not allow explicit boolean values to be provided.

The exception is --latest which uses ExplicitBooleanOptionHandler.

This change removes the misleading "set to true" or "set to false" for
the cli flags that do not accept true/false (or any other values).

This caused me a bit of trouble as I was trying to use the CLI, following the documentation, and it kept failing like so:

No argument is allowed: true
Exception in thread "main" java.io.IOException: Failed to read command-line arguments
        at io.jenkins.tools.pluginmanager.cli.Main.main(Main.java:26)
Caused by: org.kohsuke.args4j.CmdLineException: No argument is allowed: true
        at org.kohsuke.args4j.CmdLineParser.parseArgument(CmdLineParser.java:509)
        at io.jenkins.tools.pluginmanager.cli.Main.main(Main.java:22)

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[timja]

Thanks!