Update documentation of what a "section" is

Update documentation of what a "section" is

There are so many slightly different understandings of what a
"section" is or can be. I'm aware the documentation was improved
just a few weeks ago. I still find it incomplete and confusing.

1. I renamed it to $sectionId to make it more clear what it
really is.

2. Sections are usually numbers. 0, 1 and so on. There is no
reason to disallow the use of ints or even floats (this works
because the string representation of 0.0 is "0"). The code never
disallowed numbers.

3. 'T1' never was supported, as far as I can tell. 'T-1' is
supported. See Parser::extractSections().

4. null and false and '' all mean "the whole page" in
WikiPage::replaceSectionAtRev() but for some reason this meaning got
lost in WikitextContent::replaceSection(). I made it the same again.

Change-Id: Icc3997722d2ed742bf7703cd7c06d09199225720