module name misinformation

The module page has conflicting information on module names. This adds just a few words (and reorganizes when needed to keep the line length consistent) to clear it up. The module name matches the file name only by default, as affirmed later in the page, so I add the words "by default" and say it can be changed in the intro paragraph.

Later, it says "those names have a one-to-one correspondence". This is not strictly true, as we see later with "foo-bar.d" being "module foo_bar", the declared module name matching the filename is only a convention. So I added the word "can", meaning follow this convention to get the benefit, but it is not required by the language. Indeed, forcing filenames to match module names is impossible anyway, since files can have multiple names and multiple parent directories, whereas modules only have the one name!

See my exchange in this stack overflow comment thread to see what prompted this: http://stackoverflow.com/questions/20319966/how-do-you-compile-multiple-files-in-different-folders-in-d/20320021?noredirect=1#comment30366156_20320021