General grammar and style fixes in the book

Contains numerous grammar, style and readability fixes (see diff)

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | >= 2.3
| Fixed tickets | 
IMHO, the difference here lays on the logical subject:

- Subject on "the file" and "the name" as an attribute => "file name" (like "The file's name is...")
- Subject on "the name of the file" => "filename" (like "The filename is too long...")

But to make it short, the term "filename" is - as you wrote - more common, and nobody cares about this tiny difference, so I'll change it back.