SSI-related documentation requests

Hello apache2 documentation contributors,

I recently set up server-side includes on my web server today, but
spent a few hours struggling with the documentation. I have a few
suggestions on possible improvements to the documentation, to hopefully
save future readers time if they encounter the same troubles I had.

On "Introduction to Server Side Includes", Conditional expressions:
Would it be possible for the documentation to mention that, while
environment variables use %{name}, SSI vars use v('name')? I was unable
to find this on either the introduction page or the linked expressions
syntax page; instead, I found it in the documentation for mod_include,
but not as formal documentation, only as part of a single example that
does not explicitly call out the meaning of v('name'). This would allow
new users to hit the ground running with using expressions involving
SSI variables.

On "Apache Module mod_include", SSIUndefinedEcho:
Would it be possible for the documentation to mention that the value of
SSIUndefinedEcho is overridden by `#config echomsg`? Additionally,
would it possible to mention that it is not possible to set this to a
blank string, whereas the `#config` approach allows this? (This one
might be a problem between my keyboard and chair; on my setup at least,
`SSIUndefinedEcho ""` errors out as if I hadn't set any parameters.)

On "Apache Module mod_include", The echo element:
Would it be possible for the documentation to state that all variables
are allowed to be used here? The intro sentence makes it seem like this
directive only recognizes the listed Include Variables instead of all
environment and SSI-set variables.

I am happy to discuss these suggestions. My intent is to hopefully
reduce the amount of time future newbies spend troubleshooting this
functionality.

Thank you,
Hunter Turcin

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