doc: improve document of ECPG host variable

Hi hackers,

I attach a small patch improving document of ECPG host variable.
Please back patch to supported version, if possible.

Range of 'bool' as type of ECPG is not defined explicitly. Our customer was confused.
Additionally, I could not understand clearly what the existing sentence mentions to user.

My idea is as follows:

- [b] declared in ecpglib.h if not native
+ [b] Range of bool is true/false only.
There is no need to include any header like stdbool.h for type and literals
because they are defined by ECPG and
ECPG internally includes a appropriate C language standard header or
deifnes them if there is no such header.

Best Regards
Ryo Matsumura