[wip] Ulimit documentation improvements

[p-mongo]

• Adds example output of /proc/<pid>/limits
• Provides this example earlier in the document, where retrieving limits is described
• Renames "/proc file system" to "proc file system" since that is the proper name (/proc is the mount point)
• Prose edited to improve flow in my opinion

[p-mongo]

I can certainly leave this PR until the redesign, but I am having trouble understanding the following:

I think the key consideration is that ulimit values are already set in the appropriate init scripts

Whatever values may be set in the init scripts, they may need to be adjusted up (if they are set conservatively and the particular deployment has many resources) or down (if they are set greedily and the particular deployment wants to be constrained). Therefore I would think that simply saying that init scripts that come with MongoDB set the values "appropriately" is not particularly helpful for administrators who need to change those values to suit their environment.