Improve docs on pinning version numbers

[m-aciek]

Second take (see #7807) on improving Pinned Version Numbers documentation in User Guide.

• Added comment # via SomePackage as a good practice example of preserving cause of presence of package,
• added suggestion for pinning version numbers by hand using pip show,
• added suggestion for putting sub-dependencies in constraints file to avoid installing not needed package,
• added warnings about using pip freeze generation.

Although #7811 and #6925 would make those changes not needed, in my opinion they express best practices for pip's features as they are today and help its users to avoid pitfalls.

[pradyunsg]

May I suggest looking at #9474 and suggest if the changes made here are still relevant after the changes made to this section there? (this whole ensuring repeatability section becomes a dedicated page there, which I'm happy to update if we have agreement on any suggested changes to be made.

[pradyunsg]

Closing this out, since this is significantly out of date. For what it's worth, There's a dedicated page for this now: https://pip.pypa.io/en/latest/topics/repeatable-installs/

If there's still interest in doing this, I encourage the interested folks to file a new PR for this. :)