Javadoc issues

[deining]

Hi,

while reading the API docs of the newly introduced PropertiesDefaultProvider, I realized that parts of the javadoc were missing. The attached patch fixes this issue.
A second patch fixes all javadoc errors that were present in the project.

Thanks for considering my input.

[codecov-io]

Codecov Report

Merging #874 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #874   +/-   ##
=========================================
  Coverage     92.83%   92.83%           
  Complexity      388      388           
=========================================
  Files             2        2           
  Lines          5810     5810           
  Branches       1512     1512           
=========================================
  Hits           5394     5394           
  Misses          205      205           
  Partials        211      211

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/picocli/CommandLine.java	92.5% <ø> (ø)	273 <0> (ø)	⬇️

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[remkop]

Merged into master.
Thank you!