Simplified HMAC explanation

The original HMAC explanation was somewhat convoluted. I've separated the two steps more clearly, and made sure to mention every term of the formula.
Also added spacing to the outer parentheses in the formula to make it more clear where the boundary is of the outer H()

Author: jeroenh

crypto.rst

@@ -607,7 +607,7 @@ HMAC is a function of both the key and the input text. An approach to
 calculating HMACs has been standardized by NIST and takes the
 following form:
 
-HMAC = H((K⊕opad) || H((K⊕ipad) || text))
+HMAC = H( (K⊕opad) || H((K⊕ipad) || text) )
 
 H is the hash function, K is the key, and opad (output pad) and ipad
 (input pad) are well-known strings that are XORed (⊕) with the key. ||
@@ -618,12 +618,13 @@ book. However, this approach has been proved to be secure as long as
 the underlying hash function H has the appropriate
 collision-resistance properties outlined above. Note that the HMAC
 takes a hash function *H* that is not keyed, and turns it into a keyed
-hash by using the key (XORed with another string, *ipad*) as the first
-block to be fed into the hash function. The output of
-the keyed hash is then itself subjected to another keyed hash (again
-by XORing the key with a string and using that as the first block fed
-to the hash). The two passes of the keyed-hash function are important
-to the proof of security for this HMAC construction.
+hash by applying the hashing function *H* twice. First the key (XORed
+with a string, *ipad*) is prepended to the message, which is
+then fed into the hash function. The output of this keyed hash is then
+itself subjected to another keyed hash (again by XORing the key with
+a string, *opad*, and prepending that to the output of the first keyed hash).
+The two passes of the keyed-hash function are important to the proof
+of security for this HMAC construction.
 
 .. let's delete this incorrect pic for now; unclear if a new picture
    adds much