Enhance PHPdoc of Service write value parameter

[phil-davis]

Fixes #237

This makes the PHPdoc of Service write allow an array with integer keys to be passed in the value parameter.
That is passed to:
$w->writeElement($rootElementName, $value);

And Writer writeElement already has PHPdoc that allows array<int|string, mixed>

[codecov]

Codecov Report

Merging #247 (c2547e2) into master (c4cec17) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #247   +/-   ##
=========================================
  Coverage     96.90%   96.90%           
  Complexity      116      116           
=========================================
  Files            13       13           
  Lines           485      485           
=========================================
  Hits            470      470           
  Misses           15       15           

Impacted Files	Coverage Δ
lib/Service.php	98.59% <ø> (ø)

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[staabm]

lgtm