Split memory management routines into separate entries

[nspark]

This PR moves the combined entries of shmem_{malloc, free, align, realloc} into separate API entries for each. It also expands on the memory management introduction with text common across all four routines.

No semantic changes are intended. The git diff may not be very helpful. Comparing PDFs is probably the best approach.

Closes #212

[jdinan]

@nspark Please add a changelog entry so that we can merge these changes. Thanks!

[nspark]

@jdinan I pull --rebase'd to master because my branch didn't have a 1.6 changelog section at all, then added the changelog entry.

[jdinan]

Thanks!