CDRIVER-1926 fix truncated man page lines

Mallard lines like "Optional <code>bson_t</code>. Might be NULL." was
output as:

Optional
.B bson_t
. Might be NULL.

Groff misinterprets, and truncates at the first period. Escape the dot:

Optional
.B bson_t
\[char46] Might be NULL.