Documentation issue: chewing_handle_Default accepts asciis 32~127

[andy0130tw]

In the documentation libchewing.texi, it states that chewing_handle_Default only accepts alphanumeric characters as input,

https://github.com/chewing/libchewing/blob/89ef2a8453e4bca8465a3fff172a39df79f28316/doc/libchewing.texi#L306-L312

At a quick glance, it is unusual that a default function only handles a tiny set of keys. There is no other function that seems to be "generic". For example, there does not appear to be a function that specifically handles the key to open the punctuation panel `. Upon further inspection, I found the code itself showed the inconsistency, as spaces are passed in to the function in chewingio.c,

https://github.com/chewing/libchewing/blob/89ef2a8453e4bca8465a3fff172a39df79f28316/src/chewingio.c#L833

This inconsistency is also present in ibus-chewing, from which I come across. Its engine passes everything in ASCII range 32-127 to this function. It seems to be fine on my keyboard layout, though. Uncertain if it is the intended use of the function by libchewing.