A variety of minor documentation fixups

[jessey-git]

Description

A variety of minor documentation fixups:

• Use Python 3 style 'print' statements consistently everywhere (some snippets had a mix of both)
• Correct the usage of ImageSpec::channels instead of ImageSpec::nchannels
• Template syntax errors in two C++ snippets
• Document the new default for max_memory_MB

Tests

Checklist:

• I have read the contribution guidelines.
• If this is more extensive than a small change to existing code, I
  have previously submitted a Contributor License Agreement
  (individual, and if there is any way my
  employers might think my programming belongs to them, then also
  corporate).
• I have updated the documentation, if applicable.
• I have ensured that the change is tested somewhere in the testsuite
  (adding new test cases if necessary).
• My code follows the prevailing code style of this project.

[lgritz]

Thanks, very helpful!