Grammar cleanup (#875)

This commit:
* fixes inconsistencies (primarily regarding capitalization)
* fixes non-proper nouns that were improperly capitalized
* fixes reminents from not-so-meticulous copy+paste jobs

src/structures/UserProfile.js

@@ -13,14 +13,14 @@ class UserProfile {
     this.user = user;
 
     /**
-     * The Client that created the instance of the the User.
+     * The Client that created the instance of the the UserProfile.
      * @type {Client}
      */
     this.client = this.user.client;
     Object.defineProperty(this, 'client', { enumerable: false, configurable: false });
 
     /**
-     * Guilds that the ClientUser and the User share
+     * Guilds that the client user and the user share
      * @type {Collection<Guild>}
      */
     this.mutualGuilds = new Collection();