mod_proxy.xml: clarify the usage of variable interpolation

    environment variables will not exist when this interpolation happens,

    so you may still have to resort to <module>mod_rewrite</module>

    for complex rules.  Also note that interpolation is not supported

    within the scheme portion of a URL.  Dynamic determination of the

    scheme can be accomplished with <module>mod_rewrite</module> as in the

    following example.</p>

    within the scheme/hostname/port portion of a URL. Dynamic determination of

    those fields can be accomplished with <module>mod_rewrite</module>.

    The following example describes how to use <module>mod_rewrite</module>

    to dynamically set the scheme to http or https:</p>

    <highlight language="config">

RewriteEngine On

    and causes them to substitute the value of an environment

    variable <code>varname</code> for the string <code>${varname}</code>

    in configuration directives if the <var>interpolate</var> option is set.</p>

    <p>Keep this turned off (for server performance) unless you need it!</p>

    <p>The scheme/hostname/port portion of <directive>ProxyPass</directive> cannot

    contain variables, please consider using <module>mod_rewrite</module> instead.</p>

    <p>Keep this turned off (for server performance) unless you need it!

    Adding variables to <directive>ProxyPass</directive> for example may lead to

    using the default mod_proxy's workers configured (that don't allow any fine

    tuning like connections reuse, etc..).</p>

</usage>

</directivesynopsis>