Add docstrings for days of the week

[kshyatt]

Following #31218...

[fredrikekre]

How about this pattern instead:

const Monday, ...., = 1, ...
const Mon, ..., = 1, ...

"""
    Monday
    Mon

The first day of the week.

# Examples
```jldoctest
julia> Monday
1
```
"""
Mon, Monday

...

[kshyatt]

@fredrikekre I tried to match the pattern already in place for the months.

[fredrikekre]

@fredrikekre I tried to match the pattern already in place for the months.

Change that too then :) I just think it is completely unecessary to maintain two different docstrings for aliases to the integers 1-7 (and 1-12 for the months).

[StefanKarpinski]

Is it possible to attache a single doc string to the symbols for the days? It would be pretty weird for the Monday doc string to be attached to the number 1.

[dkarrasch]

x-ref #31202

[kshyatt]

@fredrikekre is this better? I checked and tests passed and docs build (and the docstring lookup works).

[kshyatt]

Win32 and Travis errors are unrelated.

[fredrikekre]

I still don't think we need separate docstrings for the long and short names, why not just

const Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, Sunday = 1, 2, 3, 4, 5, 6, 7
const Mon, Tue, Wed, Thu, Fri, Sat, Sun = 1, 2, 3, 4, 5, 6, 7

"""
    Monday
    Mon

The first day of the week.

# Examples
```jldoctest
julia> Monday
1
```
"""
Monday, Mon

"""
    Tuesday
    Tue

The second day of the week.

# Examples
```jldoctest
julia> Tuesday
2
```
"""
Tuesday, Tue

...

etc?

[kshyatt]

Now even more sm0l

[kshyatt]

Anything else we want for this?