[basic.link] Add oxford comma

[eisenwave]

The lack of commas in [basic.link] p4 really had me stumped for a few minutes because I thought it should be read as:

The name of an entity that belongs to a
    namespace scope that has not been given internal linkage above
and that is the name of ...

... when the intended reading is

The name of an entity
- that belongs to a namespace scope,
- that has not been given internal linkage above, and
- that is the name of ...

This PR adds an Oxford comma to make it immediately obvious which reading is intended.

[tkoeppe]

The PR seems to do more than add an Oxford comma, and also adds an interior comma.

@jensmaurer Is it actually clear that this is editorial? The original wording could be taken to mean that that only "namespace scopes with internal linkage" are being considered. I realize that's probably not intended, but it sounds like that could be a valid reading.

[eisenwave]

It would not be a meaningful reading (albeit grammatically valid) because [basic.link] p4.9 states:

otherwise, if the enclosing namespace has internal linkage, the name has internal linkage;

This is nonsensical if we read the whole paragraph as

[...] namespace scope that has not been given internal linkage above [...]

... because this bullet would never apply to anything.

Furthermore, the word "above" would have to apply to the sentence above, not to a prior paragraph above, which is more typical.

[jensmaurer]

@tkoeppe, I agree the other reading is possible, but it yields a meaningless statement, so I think this is one of those "pellucidly obvious" cases.

[tkoeppe]

OK, makes sense, thanks!