doc: structure of function substitutes chapter

[Bruno Haible]

Hi,

Currently, in the Gnulib manual [1], chapter 10 "ISO C and POSIX Function
Substitutes" lists all functions in alphabetical order. Whereas
chapter 13 "Glibc Function Substitutes" lists the function sorted by
header file, resulting in related function to sit close together.

I'm thinking that the same structure should also be applied to chapter 10.

Opinions? Objections?

Bruno

[1] https://www.gnu.org/software/gnulib/manual/html_node/index.html