doc: clarify N-API version Matrix

I was asked by a community member if they needed to
recompile for v12.x based on the version matrix. Add
additional context to help ensure it is clear that this
is not the case.

PR-URL: https://github.com/nodejs/node/pull/27942


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Beth Griggs <Bethany.Griggs@uk.ibm.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Christopher Hiller <boneskull@boneskull.com>
Reviewed-By: James M Snell <jasnell@gmail.com>