Add phpdoc for Symfony Command execute

[phil-davis]

Description

phan is complaining: https://drone.owncloud.com/owncloud/data_exporter/954/3/3

php -d zend.enable_gc=0 vendor-bin/phan/vendor/bin/phan --config-file .phan/config.php --require-config-exists
lib/Command/ExportUser.php:51 PhanTypeMissingReturn Method \OCA\DataExporter\Command\ExportUser::execute is declared to return int but has no return value
lib/Command/ImportUser.php:49 PhanTypeMissingReturn Method \OCA\DataExporter\Command\ImportUser::execute is declared to return int but has no return value
make: *** [test-php-phan] Error 1

This is because some Symfony4 comes in core now, and the execute method there has phpdoc that declares a return of int.

Add phpdoc the same as in other command code - e.g. from ExportInstance.php

How Has This Been Tested?

CI

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [ ] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [ ] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.

[codecov[bot]]

Codecov Report

Merging #139 into master will not change coverage. The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #139   +/-   ##
=========================================
  Coverage     86.93%   86.93%           
  Complexity      202      202           
=========================================
  Files            32       32           
  Lines           949      949           
=========================================
  Hits            825      825           
  Misses          124      124

Impacted Files	Coverage Δ	Complexity Δ
lib/Command/ImportUser.php	80% <ø> (ø)	4 <0> (ø)	:arrow_down:
lib/Command/ExportUser.php	84.21% <ø> (ø)	4 <0> (ø)	:arrow_down:

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Continue to review full report at Codecov.

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