refactor: cleanups post unsubtree'ing univalue

Mostly changes to remove src/univalue exceptions from the various linters,
and the required code changes to make them happy. As well as minor doc
changes.

test/lint/lint-shell.py

@@ -68,7 +68,7 @@ def main():
     ]
     files = get_files(files_cmd)
     # remove everything that doesn't match this regex
-    reg = re.compile(r'src/[leveldb,secp256k1,minisketch,univalue]')
+    reg = re.compile(r'src/[leveldb,secp256k1,minisketch]')
     files[:] = [file for file in files if not reg.match(file)]
 
     # build the `shellcheck` command