Address review comments

bip-wallet-policies.mediawiki

@@ -201,6 +201,8 @@ Each element of the key origin information vector is a <tt>KEY</tt> expression.
 
 The wallet policy is invalid if any placeholder expression with additional derivation steps is used when the corresponding key information is not an xpub.
 
+A wallet policy must have at least one key placeholder and the corresponding key.
+
 The key information vector should be ordered so that placeholder <tt>@i</tt> never appear for the first time before an occurrence of <tt>@j</tt>  for some j < i</tt>; for example, the first placeholder is always <tt>@0</tt>, the next one is <tt>@1</tt>, etc.
 
 === Descriptor derivation ===
@@ -229,7 +231,7 @@ Avoiding key reuse among different wallet accounts is also extremely important,
 
 == Examples ==
 
-In the examples in this section, the vector of key information items is omitted. See the test vectors below for a complete
+In the examples in this section, the vector of key information items is omitted. See the test vectors below for complete examples.
 
 Common single-signature account patterns:
 * <tt>pkh(@0/**)</tt> (legacy).
@@ -239,10 +241,10 @@ Common single-signature account patterns:
 
 Common multisignature schemes:
 * <tt>wsh(multi(2,@0,@1))</tt> - SegWit 2-of-2 multisignature, keys in order.
-* <tt>sh(sortedmulti(2,@0,@1,@3))</tt> - Legacy 2-of-3 multisignature, sorted keys.
+* <tt>sh(sortedmulti(2,@0,@1,@2))</tt> - Legacy 2-of-3 multisignature, sorted keys.
 
 Some miniscript policies in <tt>wsh</tt>:
-* <tt>wsh(and_v(v:pk(key_user),or_d(pk(key_service),older(12960))))</tt> - Trust-minimized second factor, degrading to a single signature after about 90 days.
+* <tt>wsh(and_v(v:pk(@0),or_d(pk(@1),older(12960))))</tt> - Trust-minimized second factor, degrading to a single signature after about 90 days.
 * <tt>wsh(thresh(3,pk(@0/**),s:pk(@1/**),s:pk(@2/**),sln:older(12960)))</tt> - A 3-of-3 wallet that becomes a 2-of-3 if coins are not spent for about 90 days.
 * <tt>wsh(t:or_c(pk(@0/**),and_v(v:pk(@1/**),or_c(pk(@2/**),v:hash160(00112233445566778899aabbccddeeff00112233)))))</tt> - BOLT #3 offered HTLC policy.
 * <tt>wsh(andor(pk(@0/**),or_i(and_v(v:pkh(@1/**),hash160(00112233445566778899aabbccddeeff00112233)),older(1008)),pk(@2/**)))</tt> - BOLT #3 received HTLC policy.
@@ -295,7 +297,7 @@ TBD: add examples with taproot scripts and miniscript.
 
 == Backwards Compatibility ==
 
-The <tt>@</tt> character used for key placeholders is not part of the syntax of output script descriptors, therefore any valid descriptor template is not a valid descriptor template. Vice versa, any descriptor template with at least one key placeholder is not a valid output script descriptor.
+The <tt>@</tt> character used for key placeholders is not part of the syntax of output script descriptors, therefore any valid output descriptor with at least one `KEY` expression is not a valid descriptor template. Vice versa, any descriptor template with at least one key placeholder is not a valid output script descriptor.
 
 Adoption of wallet policies in software and harder wallets is opt-in. Conversion from wallet policies to the corresponding descriptors is programmatically extremely easy, and conversion from descriptors to wallet policies (when respecting the required patterns) can be automated. See the reference implementation below for some examples of conversion.