Extend --with-zstd documentation

The patch that was so far added for documenting --with-zstd is pretty
minimal:
  - it refers to undocumented options --with-zstd-include and
    --with-zstd-lib;
  - it suggests that --with-zstd can be used without an argument;
  - it does not clarify how this option applies to cross-compilation.

How about adding the same details as for the --with-isl,
--with-isl-include, --with-isl-lib options, mutatis mutandis? This patch
does that.

	PR other/105527

gcc/ChangeLog:

	* doc/install.texi (Configuration): Add more details about --with-zstd.
	Document --with-zstd-include and --with-zstd-lib

Signed-off-by: Bruno Haible <bruno@clisp.org>
(cherry picked from commit 3677eb80b6)

gcc/doc/install.texi

@@ -2372,10 +2372,20 @@ default is derived from glibc's behavior. When glibc clamps float_t to double,
 GCC follows and enables the option. For other cross compiles, the default is
 disabled.
 
-@item --with-zstd
-Specify prefix directory for installed zstd library.
-Equivalent to @option{--with-zstd-include=PATH/include} plus
-@option{--with-zstd-lib=PATH/lib}.
+@item --with-zstd=@var{pathname}
+@itemx --with-zstd-include=@var{pathname}
+@itemx --with-zstd-lib=@var{pathname}
+If you do not have the @code{zstd} library installed in a standard
+location and you want to build GCC, you can explicitly specify the
+directory where it is installed (@samp{--with-zstd=@/@var{zstdinstalldir}}).
+The @option{--with-zstd=@/@var{zstdinstalldir}} option is shorthand for
+@option{--with-zstd-lib=@/@var{zstdinstalldir}/lib} and
+@option{--with-zstd-include=@/@var{zstdinstalldir}/include}. If this
+shorthand assumption is not correct, you can use the explicit
+include and lib options directly.
+
+These flags are applicable to the host platform only.  When building
+a cross compiler, they will not be used to configure target libraries.
 @end table
 
 @subheading Cross-Compiler-Specific Options