[doc] clarify the edge cases where the input buffer is empty

[gfx]

As JSON.parse("") raises a SyntaxError, msgpack.decode([]) raises a RangeError. In fact, it always raises RangeError if the input is insufficient.

resolve #198

[codecov-commenter]

Codecov Report

Merging #209 (bebb16b) into main (b17287d) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #209   +/-   ##
=======================================
  Coverage   98.13%   98.13%           
=======================================
  Files          16       16           
  Lines         964      964           
  Branches      206      206           
=======================================
  Hits          946      946           
  Misses         18       18           

Impacted Files	Coverage Δ
src/Decoder.ts	99.16% <ø> (ø)
src/decode.ts	100.00% <ø> (ø)
src/decodeAsync.ts	90.47% <100.00%> (ø)

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[gfx]

On the other hand, decodeMulti*() can decode empty inputs, generating an empty stream or array. 2b66bb7