fixed validation for "Exclude products" in coupons metabox, fixes #4509
fixed the indentation in includes/admin/post-types/meta-boxes/class-wc-meta-box-coupon-data.php
fixed some coding standards in includes/admin/post-types/meta-boxes/class-wc-meta-box-coupon-data.php
Merge pull request #4513 from splashingpixels/master

added coupon text for clarity and coding standards fixes
Merge pull request #4512 from claudiosmweb/fix_4509

Fix #4509