ck_hs: Add ck_hs_reset_size operation.

The ck_hs_reset_size operation takes an additional argument
that allows the user to specify the size of the new generation
of the hashset.
ck_ht: Add ck_ht_reset_size_spmc operation.

The ck_ht_reset_size_spmc operation takes an additional argument
that allows the user to specify the size of the new generation
of the hashtable.
doc: Add ck_hs_reset_size manual page.
doc: Cross-reference ck_hs_reset_size and fix title.
doc: Add ck_ht_reset_size_spmc manual page, fix references.

This commit also fixes incorrect references to non-existent
ck_ht_reset function (replaced with ck_ht_reset_spmc).