revise option specification to control include paths

[mycoboco]

The current short explanation for controlling #include paths is incorrect and needs to be revised.

[mycoboco]

To sum up beluga's behavior,

• beluga supports -I, -isystem and -idirafter;
• -I adds to the user include path;
• -isystem prepends to the system include path; and
• -idirafter appends to the system include path.

The order in which paths are taken is:

• current ("..."-search starts here)
• -I (<...>-search starts here)
• -isystem (system headers from here)
• preconfigured
• -idirafter

To compare to gcc:

• beluga has no support for -iquote and = replacement.

gcc's #include path order is:

• current ("..."-search starts here)
• -iquote
• -I (<...>-search starts here)
• -isystem (system headers from here)
• preconfigured
• -idirafter