Apply latest cs-fixer changes

[phil-davis]

The latest cs-fixer does not like having "useless" PHP doc that declares a var or param to be mixed.
But phpstan complains when the param has no type declaration.

The workaround is to keep the mixed PHPdoc declarations, and add some text description to them. That makes cs-fixer think that they are not useless.

It's a bit annoying having to do this, but it is not completely stupid and does get both tools to pass.

[codecov]

Codecov Report

Merging #109 (08151ed) into master (fea6ea5) will not change coverage.
The diff coverage is n/a.

❗ Current head 08151ed differs from pull request most recent head 1ee3b49. Consider uploading reports for the commit 1ee3b49 to get more accurate results

@@            Coverage Diff            @@
##             master     #109   +/-   ##
=========================================
  Coverage     99.73%   99.73%           
  Complexity      109      109           
=========================================
  Files             7        7           
  Lines           376      376           
=========================================
  Hits            375      375           
  Misses            1        1           

Impacted Files	Coverage Δ
lib/Promise.php	100.00% <ø> (ø)
lib/Promise/functions.php	100.00% <ø> (ø)

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