docs: update examples with += env operator and env usage docs

- foo.kap: document all three env operators (=, ?=, +=) in header,
  demonstrate += in env block with comments

- postgres.kap: demonstrate += for appending security hardening flags

- config.kap: demonstrate system-level += for inherited flags

examples/config.kap

@@ -67,6 +67,8 @@ system {
 
     env {
         CFLAGS    = "-O2 -march=native"
+        // packages inherit this appended flag
+        CFLAGS   += "-pipe"
         LDFLAGS   = "-Wl,--as-needed"
         MAKEFLAGS = "-j8"
     }