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/postgres.kap

@@ -77,6 +77,8 @@ package "postgresql" {
 
     env {
         CFLAGS  = "-O2"
+        // append security hardening flags
+        CFLAGS += "-D_FORTIFY_SOURCE=2"
         LDFLAGS = "-Wl,--as-needed"
     }