[fortran,doc] Mention library problems with -ffpe-trap=precision

[FX Coudert]

Hi all,

This is the doc patch I intend to commit to close PR30780 (and fix  
two typos). I'm not a native speaker, so I welcome any suggestion to  
make this sentence better. I'll commit in a few days.

Thanks,
FX



Index: invoke.texi
===================================================================
--- invoke.texi (revision 128673)
+++ invoke.texi (working copy)
@@ -550,6 +550,11 @@ zero), @samp{overflow} (overflow in a fl
@samp{precision} (loss of precision during operation) and @samp 
{denormal}
(operation produced a denormal value).
+Some of the routines in the Fortran runtime library, like
+@samp{CPU_TIME}, are likely to to trigger floating point exceptions  
when
+@code{ffpe-trap=precision} is used. For this reason, the use of
+@code{ffpe-trap=precision} is not recommended.
+
@item -fbacktrace
@opindex @code{fbacktrace}
@cindex backtrace
@@ -775,8 +780,8 @@ tools.
Use of @option{-fno-underscoring} is not recommended unless you are
experimenting with issues such as integration of GNU Fortran into
-existing system environments (vis-a-vis existing libraries, tools, and
-so on).
+existing system environments (vis-@`{a}-vis existing libraries, tools,
+and so on).
For example, with @option{-funderscoring}, and assuming other  
defaults like
@option{-fcase-lower} and that @code{j()} and @code{max_count()} are
@@ -862,7 +867,7 @@ checks array indices for assumed and def
shape arrays against the actual allocated bounds.
Some checks require that @option{-fbounds-check} is set for
-the compilation of the main probram.
+the compilation of the main program.
In the future this may also include other forms of checking, e.g.,  
checking
substring references.