Update imshow docs.

[anntzer]

• Reword a bit description of X argument. Note that any integer
  data, even if not uint8, is interpreted as 0-255 uint8 (with clipping
  to that range).
• Move the doc for norm up to its position in the signature (right
  after cmap). Reword the docs for cmap and norm to be phrased
  more similarly.
• Move the doc for alpha up to its position in the signature (before
  vmin, vmax).

PR Summary

PR Checklist

• Has Pytest style unit tests
• Code is Flake 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way