Update logging.jl to add docstrings for exported symbols

[AllHailTheSheep]

This is a part of issue #52725 and supersedes this pull request as I specified the incorrect base repo.

[LilithHafner]

Thank you!

These symbols are internal, but documenting internals is good. This PR changes

before:

help?> Base.CoreLogging.AboveMaxLevel
  │ Warning
  │
  │  The following bindings may be
  │  internal; they may change or be
  │  removed in future versions:
  │
  │    •  Base.CoreLogging
  │
  │    •  Base.CoreLogging.AboveMaxLevel

  No documentation found for private symbol.

  Base.CoreLogging.AboveMaxLevel is of type
  Base.CoreLogging.LogLevel.

  Summary
  ≡≡≡≡≡≡≡

  struct Base.CoreLogging.LogLevel

  Fields
  ≡≡≡≡≡≡

  level :: Int32

to

after:

help?> Base.CoreLogging.AboveMaxLevel
  │ Warning
  │
  │  The following bindings may be
  │  internal; they may change or be
  │  removed in future versions:
  │
  │    •  Base.CoreLogging
  │
  │    •  Base.CoreLogging.AboveMaxLevel

  AboveMaxLevel

  Alias for LogLevel(1_000_001).

Which is an unambiguous improvement. (With the public keyword it is clear that adding these docstrings does not make these symbols public)