Update doc

Author: WeiqunZhang

Docs/sphinx_documentation/source/GPU.rst

@@ -564,7 +564,11 @@ parameter, ``amrex.the_arena_init_size``, in the unit of bytes. The default
 can also be changed with an environment variable
 ``AMREX_THE_ARENA_INIT_SIZE=X``, where ``X`` is the number of bytes. When
 both the :cpp:`ParmParse` parameter and the environment variable are
-present, the former will override the latter.
+present, the former will override the latter. In both cases, the number
+string could have optional single quotes ``'`` as separators (e.g.,
+``10'1000'100'``). It may also use floating-point notation (``2.5e10``), as
+long as converting it does not introduce any loss of precision.
+
 The default initial size for other arenas is 8388608 (i.e., 8 MB).  For
 :cpp:`The_Managed_Arena()` and :cpp:`The_Device_Arena()`, it can be changed
 with ``amrex.the_managed_arena_init_size`` and