Document magic numbers in Printf for Floats

[Seelengrab]

For Ints and Chars (in the code above the one I link) it's somewhat more straightforward what the numbers mean and why they're chosen, but for Floats it's kind of guesswork unless you already know what they mean. This came up while investigating #41631

julia/stdlib/Printf/src/Printf.jl

Lines 779 to 782 in 6279f3b

	plength(f::Spec{T}, x::AbstractFloat) where {T <: Ints} =
	max(f.width, 0 + 309 + 17 + f.hash + 5)
	plength(f::Spec{T}, x) where {T <: Floats} =
	max(f.width, f.precision + 309 + 17 + f.hash + 5)

[Seelengrab]

Just thought of looking at git blame for who to best ask about this, seems @quinnj may know? I'd be happy to make a PR once I'm sure I understand what each number means and/or where it comes from.

[quinnj]

Hmmm, I'm not sure I quite remember where those exact numbers came from; roughly it's the total number of significant digits we're willing to print/allow for precision for floats.