This one is the simplest of all if you have the locateprogramon your system.
In your terminal/console, enter the following command:
locate <filename.h>
You might also get a lot of output from this since your system might have many versions of these header files.
Method 2 is best because it tells you exactly which header file the compiler is using.
Once you have found the function you want to know more about in one of these files, you can then use the Unix man command to read about it on your system. To do so, enter the following into a terminal/console:
man 3 <function>
This tells man to look in section 3 for the given function. Section 3 is where C functions are described.
Alternatively, you could try the following:
man 7 <topic>
Section 7 is where general topics are described. There is a lot of information there...