Bug 18058 - Document to warn of issues using lm() with summary functions in the formula

[hturner]

Bug 18058 describes some behaviour that the user finds unexpected, which is due to using the subset argument with a model using summary functions in the formula (mean(x)) in the provided example.

The problems with doing this are discussed on Bugzilla.

@mmaechler suggests this bug report could be resolved by improving the documentation, by adding a note that using summary functions in the formula is a bad idea!

[TanguyBarthelemy]

We provide a patch (see r-devel/r-svn#201) and answered the issues on Bugzilla (https://bugs.r-project.org/show_bug.cgi?id=18058#c4)
with @wlangera, @jgodet and @brehel1