Update doctests for Context changes

Due to the Context having many methods removed, the doctests were
failing.

Update the doctests to use alternative methods to accomplish the same.

Example:

Before:
```rust
context.say("hi");
```

After:
```rust
message.channel_id.say("hi")
```

3 files changed, 12 insertions, 14 deletions

diff --git a/src/ext/cache/mod.rs b/src/ext/cache/mod.rs
index 064add0..b39f5ea 100644
--- a/src/ext/cache/mod.rs
+++ b/src/ext/cache/mod.rs
@@ -368,7 +368,7 @@ impl Cache {
     /// let channel = match cache.get_guild_channel(message.channel_id) {
     ///     Some(channel) => channel,
     ///     None => {
-    ///         if let Err(why) = context.say("Could not find guild's channel data") {
+    ///         if let Err(why) = message.channel_id.say("Could not find guild's channel data") {
     ///             println!("Error sending message: {:?}", why);
     ///         }
     ///
@@ -414,14 +414,12 @@ impl Cache {
     /// ```rust,ignore
     /// use serenity::client::CACHE;
     ///
-    /// // assuming you are in a context
-    ///
     /// let cache = CACHE.read().unwrap();
     /// let member = {
     ///     let channel = match cache.get_guild_channel(message.channel_id) {
     ///         Some(channel) => channel,
     ///         None => {
-    ///             if let Err(why) = context.say("Error finding channel data") {
+    ///             if let Err(why) = message.channel_id.say("Error finding channel data") {
     ///                 println!("Error sending message: {:?}", why);
     ///             }
     ///         },
@@ -430,7 +428,7 @@ impl Cache {
     ///     match cache.get_member(channel.guild_id, message.author.id) {
     ///         Some(member) => member,
     ///         None => {
-    ///             if let Err(why) = context.say("Error finding member data") {
+    ///             if let Err(why) = message.channel_id.say("Error finding member data") {
     ///                 println!("Error sending message: {:?}", why);
     ///             }
     ///         },
@@ -439,7 +437,7 @@ impl Cache {
     ///
     /// let msg = format!("You have {} roles", member.roles.len());
     ///
-    /// if let Err(why) = context.say(&msg) {
+    /// if let Err(why) = message.channel_id.say(&msg) {
     ///     println!("Error sending message: {:?}", why);
     /// }
     /// ```
diff --git a/src/ext/framework/create_command.rs b/src/ext/framework/create_command.rs
index 357a597..68319af 100644
--- a/src/ext/framework/create_command.rs
+++ b/src/ext/framework/create_command.rs
@@ -54,8 +54,8 @@ impl CreateCommand {
     ///         .desc("Replies to a ping with a pong")
     ///         .exec(ping)));
     ///
-    /// fn ping(context: &mut Context, _message: &Message, _args: Vec<String>) -> Result<(), String> {
-    ///     let _ = context.say("Pong!");
+    /// fn ping(_context: &mut Context, message: &Message, _args: Vec<String>) -> Result<(), String> {
+    ///     let _ = message.channel_id.say("Pong!");
     ///
     ///     Ok(())
     /// }
diff --git a/src/ext/framework/mod.rs b/src/ext/framework/mod.rs
index 3c926a7..552ab93 100644
--- a/src/ext/framework/mod.rs
+++ b/src/ext/framework/mod.rs
@@ -43,12 +43,12 @@
 //!     .command("about", |c| c.exec_str("A simple test bot"))
 //!     .command("ping", |c| c.exec(ping)));
 //!
-//! command!(about(context) {
-//!     let _ = context.say("A simple test bot");
+//! command!(about(_context, message) {
+//!     let _ = message.channel_id.say("A simple test bot");
 //! });
 //!
-//! command!(ping(context) {
-//!     let _ = context.say("Pong!");
+//! command!(ping(_context, message) {
+//!     let _ = message.channel_id.say("Pong!");
 //! });
 //! ```
 //!
@@ -703,8 +703,8 @@ impl Framework {
     ///     .on("ping", ping)
     ///     .set_check("ping", owner_check));
     ///
-    /// command!(ping(context) {
-    ///     let _ = context.say("Pong!");
+    /// command!(ping(_context, message) {
+    ///     let _ = message.channel_id.say("Pong!");
     /// });
     ///
     /// fn owner_check(_context: &mut Context, message: &Message) -> bool {