feat: adds some test cases where they were missing, and some minor readme additions

testdata/restore__merge__ok.ct

@@ -0,0 +1,9 @@
+# Merge import updates existing entries and adds new ones
+$ pda set existing@mrg old-value
+$ fecho dumpfile {"key":"existing","value":"updated","encoding":"text"} {"key":"new","value":"hello","encoding":"text"}
+$ pda restore mrg --file dumpfile
+  ok  restored 2 entries into @mrg
+$ pda get existing@mrg
+updated
+$ pda get new@mrg
+hello