doc: more details about symbols in m4

[Akim Demaille]

commit 238692ad77e7bdceaadc400e391783397855bb87
Author: Akim Demaille <akim.demaille@gmail.com>
Date:   Sat Jun 20 09:09:26 2020 +0200

    doc: more details about symbols in m4
    
    * data/README.md: here.
    * README-hacking.md (Vocabulary): More.

diff --git a/README-hacking.md b/README-hacking.md
index 533b5e35..e128efb3 100644
--- a/README-hacking.md
+++ b/README-hacking.md
@@ -211,6 +211,8 @@ assert/abort), and all the --trace output which is meant 
for the maintainers
 only.
 
 ## Vocabulary
+- "lookahead", not "look-ahead".
+- "midrule", not "mid-rule".
 - "nonterminal", not "variable" or "non-terminal" or "non terminal".
   Abbreviated as "nterm".
 - "shift/reduce" and "reduce/reduce", not "shift-reduce" or "shift reduce",
diff --git a/data/README.md b/data/README.md
index a1ad397f..50fbe525 100644
--- a/data/README.md
+++ b/data/README.md
@@ -86,7 +86,7 @@ The macro `b4_symbol(NUM, FIELD)` gives access to the 
following FIELDS:
 - `has_id`: 0 or 1
   Whether the symbol has an `id`.
 
-- `id`: string
+- `id`: string (e.g., `exp`, `NUM`, or `TOK_NUM` with api.token.prefix)
   If `has_id`, the name of the token kind (prefixed by api.token.prefix if
   defined), otherwise empty.  Guaranteed to be usable as a C identifier.
   This is used to define the token kind (i.e., the enum used by the return
@@ -105,9 +105,9 @@ The macro `b4_symbol(NUM, FIELD)` gives access to the 
following FIELDS:
 - `is_token`: 0 or 1
   Whether this is a terminal symbol.
 
-- `kind_base`: string
+- `kind_base`: string (e.g., `YYSYMBOL_exp`, `YYSYMBOL_NUM`)
   The base of the symbol kind, i.e., the enumerator of this symbol (token or
-  nonterminal) which is mapping to its `number`.
+  nonterminal) which is mapped to its `number`.
 
 - `kind`: string
   Same as `kind_base`, but possibly with a prefix in some languages.  E.g.,