document qmake's functions

The following qmake functions are undocumented.
All of them are implemented in qtbase/qmake/libary/qmakebuiltins.cpp

• first
• last
• size
• cat
• fromfile
• list
• format_number
• split
• section
• reverse
• quote
• escape_expand
• upper
• lower
• re_escape
• val_escape
• files
• sort_depends
• resolve_depends
• enumerate_vars
• shadowed
• absolute_path
• relative_path
• clean_path
• system_path
• shell_path
• system_quote
• shell_quote
• requires
• greaterThan
• lessThan
• equals
• isEqual
• export
• clear
• unset
• if
• isActiveConfig
• defined
• load
• debug
• log
• message
• warning
• error
• mkpath
• write_file
• touch
• cache