[Rd] logic tweak needed for as.data.frame.<class> deprecation warning

[Mikael Jagan]

Continuing the thread started on R-package-devel, here:
https://stat.ethz.ch/pipermail/r-package-devel/2023q3/009307.html

The logic of the now soft-deprecated as.data.frame.<class>,

 > body(as.data.frame.integer)[[2L]]
if ((sys.nframe() <= 1L || sys.call(-1L)[[1L]] != quote(as.data.frame)) &&
     nzchar(Sys.getenv("_R_CHECK_AS_DATA_FRAME_EXPLICIT_METHOD_"))) 
.Deprecated(msg = gettextf("Direct call of '%s()' is deprecated.  Use '%s()' or 
'%s()' instead",
     "as.data.frame.integer", "as.data.frame.vector", "as.data.frame"))

may need adjustment to avoid false positives such as this one:

 > Sys.setenv("_R_CHECK_AS_DATA_FRAME_EXPLICIT_METHOD_" = TRUE)
 > f <- as.data.frame
 > f(0L)
   0L
1  0
Warning message:
Direct call of 'as.data.frame.integer()' is deprecated.  Use 
'as.data.frame.vector()' or 'as.data.frame()' instead

i.e., the condition sys.call(-1L)[[1L]] != quote(as.data.frame) is not precise
enough ... would !identical(sys.function(-1L), as.data.frame) work instead?

Mikael