mwdocgen: support multiple --file values

It is sometimes useful to generate documentation for multiple files or
directories. Doxygen INPUT statement supports multiple space separated
paths. Example usage:

 php maintenance/mwdocgen.php --file docs,input

Change-Id: I66a805d80307c6502e9b440df63a37bb13093d84
Merge "mwdocgen: support multiple --file values"