Add documentation

* `:open`
* `:buffer-close`
* `:buffer-close!`
* `:write`
* `:new`

runtime/help/typable-commands/buffer-close!.txt

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+`:buffer-close!`
+
+Aliases: `:bc!`, `:bclose!`
+
+Closes the current buffer, discarding any
+unsaved changes to the buffer.

runtime/help/typable-commands/buffer-close.txt

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+`:buffer-close`
+
+Aliases: `:bc`, `:bclose`
+
+Closes the current buffer, failing if the buffer
+has unsaved changes.

runtime/help/typable-commands/new.txt

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+`:new`
+
+Aliases: `:n`
+
+Creates and switches to a new scratch buffer. This
+buffer will not be associated with any file path.

runtime/help/typable-commands/open.txt

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+`:open <path>`
+
+Aliases: `:o`
+
+If <path> exists, opens the file at <path>.
+If <path> does not exist, creates a new buffer
+and associates it with <path>. Note that this
+does not automatically save the created buffer.

runtime/help/typable-commands/write.txt

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+`:write [path]`
+
+Aliases: `:w`
+
+Writes (saves) the buffer to disk.
+If [path] is supplied, that path is written to.
+If the buffer is a scratch buffer, meaning it
+is not already bound to a file path, [path]
+becomes required for this command to succeed.