Rename documentation references of 'compilation unit' to 'source file'.

[megri]

This includes everything under the /docs folder and nothing else: sources and scaladoc still refer to 'compilation unit'

[som-snytt]

It's OK for popular literature, but if they took my idea to take a directory of files as a single compilation unit, then obviously the equivalence would fail. (People complained that if companions have to be in a single file, then the file gets too big. Why can't it be two files.)

[odersky]

OK with the change for user-facing docs but let's keep "compilation unit" for old blog posts and for docs in internals. The compiler has CompilationUnits and SourceFiles and they are different. Misnaming them is bound to confuse.

So I'd keep only the changes under docs/reference and docs/usage.

[megri]

OK with the change for user-facing docs but let's keep "compilation unit" for old blog posts and for docs in internals. The compiler has CompilationUnits and SourceFiles and they are different. Misnaming them is bound to confuse.

So I'd keep only the changes under docs/reference and docs/usage.

Updated now