Issue 1625 - Improve string descriptions, addressing comments on CrowdIn

_locales/en_US/messages.json

 {
   "add": {
     "description": "This is the label for the 'Add' buttons.",
     "message": "Add"
   },
   "add_domain_button": {
     "message": "Add domain"
   },
   "add_filter_button": {
     "message": "Add filter"
   },
   "add_filters_msg": {
     "message": "Add filter(s)?"
   },
   "add_your_own_filters": {
     "message": "Add your own filters"
   },
   "apply_changes": {
     "message": "Apply changes"
   },
   "block_element": {
     "description": "Action in context menu and popup UI, to add a filter which blocks an element selected from the current page",
     "message": "Block element"
   },
   "cancel": {
     "description": "Cancel button label",
     "message": "Cancel"
   },
   "clickhide_instructions": {
+    "description": "Instructions shown in popup UI before it disapears, when \"Block element\" action is executed",
     "message": "After closing this popup, click (or right-click) an element on the page."
   },
   "description_chrome": {
     "description": "Note: this cannot be longer than 132 characters",
     "message": "The free adblock tool for Chrome: Blocks annoying video ads on YouTube, Facebook ads, banners and much more."
   },
   "description_opera": {
     "message": "The free adblock tool for Opera: Blocks annoying video ads on YouTube, Facebook ads, banners and much more."
   },
   "description_safari": {
@@ skipping 113 matching lines @@
   "stats_share_message": {
     "description": "This message will be posted to social networks if the user decides to share the stats.",
     "message": "I blocked $number$ ads and trackers thanks to Adblock Plus.",
     "placeholders": {
       "number": {
         "content": "$1"
       }
     }
   },
   "stats_share_title": {
-    "description": "Title for the share buttons to share stats.",
+    "description": "Label for social media buttons sharing the number of blocked ads",
     "message": "Share these numbers"
   },
   "stats_show_iconnumber": {
     "description": "Toggle for displaying the number of blocked ads in the icon.",
     "message": "Show number in icon"
   },
   "stats_title": {
     "description": "Title for the stats section in the popup.",
     "message": "Ads blocked"
   },
   "subscriptionSelection_subscriptionSelector_label": {
     "message": "Please choose a filter subscription from the list:"
   },
   "update_now": {
     "message": "Update now"
   },
   "whitelisted_domains": {
     "description": "This is a tab heading.",
     "message": "Whitelisted domains"
   },
   "unexpected_filter_list_header": {
+    "description": "Error message shown when the user attempts to add a custom filter that has the format of a header (e.g. \"[Adblock Plus 2.0]\") which are only allowed in filter lists",
     "message": "Filter list headers aren't allowed here"
   },
   "invalid_css_selector": {
     "message": "$selector$ is not a valid CSS selector",
     "placeholders": {
       "selector": {
         "content": "$1"
       }
     }
   },
   "line": {
     "message": "Line $number$",
     "description": "Prepended to error messages to indicate the line number (e.g. \"Line 42: Some error on this line\")",
     "placeholders": {
       "number": {
         "content": "$1"
       }
     }
   }
 }