blockchain: Comment concurrency semantics.

This calls out the concurrency semantics for some exported query
functions to ensure callers are aware they are safe for concurrent
access to be more consistent with other exported methods in the package.

blockchain/chainquery.go

@@ -76,6 +76,8 @@ type ChainTipInfo struct {
 
 // ChainTips returns information, in JSON-RPC format, about all of the currently
 // known chain tips in the block index.
+//
+// This function is safe for concurrent access.
 func (b *BlockChain) ChainTips() []ChainTipInfo {
 	b.index.RLock()
 	chainTips := make([]*blockNode, 0, b.index.totalTips)
@@ -136,6 +138,8 @@ func (b *BlockChain) ChainTips() []ChainTipInfo {
 
 // BestHeader returns the header with the most cumulative work that is NOT
 // known to be invalid.
+//
+// This function is safe for concurrent access.
 func (b *BlockChain) BestHeader() (chainhash.Hash, int64) {
 	b.index.RLock()
 	header := b.index.bestHeader
@@ -146,6 +150,8 @@ func (b *BlockChain) BestHeader() (chainhash.Hash, int64) {
 // BestInvalidHeader returns the header with the most cumulative work that is
 // known to be invalid.  It will be a hash of all zeroes if there is no such
 // header.
+//
+// This function is safe for concurrent access.
 func (b *BlockChain) BestInvalidHeader() chainhash.Hash {
 	var hash chainhash.Hash
 	b.index.RLock()