Update docs on the GNU Q suffix.

[CI SKIP]
Fixes #148.

doc/multiprecision.qbk

@@ -1029,7 +1029,8 @@ as a valid floating-point number.
 * When using the Intel compiler, the underlying type defaults to `__float128` if it's available and `_Quad` if not.  You can override
 the default by defining either `BOOST_MP_USE_FLOAT128` or `BOOST_MP_USE_QUAD`.
 * When the underlying type is Intel's `_Quad` type, the code must be compiled with the compiler option `-Qoption,cpp,--extended_float_type`.
-* When compiling with `gcc`, you need to use the flag `--std=gnu++11/14/17`, as the numeric literal operator 'operator""Q' is a GNU extension. Compilation fails with the flag `--std=c++11/14/17`.
+* When compiling with `gcc`, you need to use the flag `--std=gnu++11/14/17`, as the suffix 'Q' is a GNU extension. Compilation fails with the flag `--std=c++11/14/17`
+unless you also use `-fext-numeric-literals`.
 
 [h5 float128 example:]