document guarantee that getmetadata returns alias not copy

[jrevels]

ref #90 (comment)

I'm relying on this currently undocumented behavior in downstream code that looks like:

function assign_to_table_metadata!(table, pairs)
    m = Arrow.getmetadata(table)
    if !(m isa Dict)
        m = Dict{String,String}()
        Arrow.setmetadata!(table, m)
    end
    for (k, v) in pairs
        m[k] = v
    end
    return m
end

Seems like there's not much harm in explicitly documenting this assumption so that I can say the above code is "generic" 😁

[codecov]

Codecov Report

Merging #169 (143a27c) into main (295416d) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #169   +/-   ##
=======================================
  Coverage   81.32%   81.32%           
=======================================
  Files          25       25           
  Lines        3014     3014           
=======================================
  Hits         2451     2451           
  Misses        563      563           

Impacted Files	Coverage Δ
src/write.jl	95.69% <ø> (ø)

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