Fix spelling and gramatical errors

 * **pure overwrite**: A write operation that does not require any

   metadata or zeroing operations to perform during either submission

   or completion.

   This implies that the fileystem must have already allocated space

   This implies that the filesystem must have already allocated space

   on disk as ``IOMAP_MAPPED`` and the filesystem must not place any

   constaints on IO alignment or size.

   constraints on IO alignment or size.

   The only constraints on I/O alignment are device level (minimum I/O

   size and alignment, typically sector size).

 * The **upper** level primitive is provided by the filesystem to

   coordinate access to different iomap operations.

   The exact primitive is specifc to the filesystem and operation,

   The exact primitive is specific to the filesystem and operation,

   but is often a VFS inode, pagecache invalidation, or folio lock.

   For example, a filesystem might take ``i_rwsem`` before calling

   ``iomap_file_buffered_write`` and ``iomap_file_unshare`` to prevent