fix rst markup in doc/manual/types.rst

In rst markup, inline literals (i.e. code samples)
can't be nested inside strong emphasis (bold).
The backticks were showing in the rendered output.
document cglobal (from issue #1815), and fix deprecated unsafe_ref/assign -> unsafe_load/store! in the manual
Merge pull request #3092 from waldir/patch-1

fix rst markup in doc/manual/types.rst