Merge bitcoin-core/secp256k1#1030: doc: Fix upper bounds + cleanup in field_5x52_impl.h comment

1287786 doc: Add comment to top of field_10x26_impl.h (Elliott Jin)
58da5bd doc: Fix upper bounds + cleanup in field_5x52_impl.h comment (Elliott Jin)

Pull request description:

  When reviewing bitcoin#816 I noticed the upper bounds in the comment at the top of `field_5x52_impl.h` were off by 1 (see `fe_verify`). This PR fixes the upper bounds and also cleans up the comment along the way.

ACKs for top commit:
  real-or-random:
    ACK 1287786

Tree-SHA512: 4b7dadc92451ab1ceb5a547a3101ff37f3ffd0645490563f1f3442ea8d6219f100ed914289d22435c4172d190fa1ff52e37e4464132bb3f9bbcc338488227f7b

Author: real-or-random

src/field_10x26_impl.h

@@ -11,6 +11,15 @@
 #include "field.h"
 #include "modinv32_impl.h"
 
+/** See the comment at the top of field_5x52_impl.h for more details.
+ *
+ *  Here, we represent field elements as 10 uint32_t's in base 2^26, least significant first,
+ *  where limbs can contain >26 bits.
+ *  A magnitude M means:
+ *  - 2*M*(2^22-1) is the max (inclusive) of the most significant limb
+ *  - 2*M*(2^26-1) is the max (inclusive) of the remaining limbs
+ */
+
 #ifdef VERIFY
 static void secp256k1_fe_verify(const secp256k1_fe *a) {
     const uint32_t *d = a->n;

src/field_5x52_impl.h

@@ -22,11 +22,18 @@
 #endif
 
 /** Implements arithmetic modulo FFFFFFFF FFFFFFFF FFFFFFFF FFFFFFFF FFFFFFFF FFFFFFFF FFFFFFFE FFFFFC2F,
- *  represented as 5 uint64_t's in base 2^52. The values are allowed to contain >52 each. In particular,
- *  each FieldElem has a 'magnitude' associated with it. Internally, a magnitude M means each element
- *  is at most M*(2^53-1), except the most significant one, which is limited to M*(2^49-1). All operations
- *  accept any input with magnitude at most M, and have different rules for propagating magnitude to their
- *  output.
+ *  represented as 5 uint64_t's in base 2^52, least significant first. Note that the limbs are allowed to
+ *  contain >52 bits each.
+ *
+ *  Each field element has a 'magnitude' associated with it. Internally, a magnitude M means:
+ *  - 2*M*(2^48-1) is the max (inclusive) of the most significant limb
+ *  - 2*M*(2^52-1) is the max (inclusive) of the remaining limbs
+ *
+ *  Operations have different rules for propagating magnitude to their outputs. If an operation takes a
+ *  magnitude M as a parameter, that means the magnitude of input field elements can be at most M (inclusive).
+ *
+ *  Each field element also has a 'normalized' flag. A field element is normalized if its magnitude is either
+ *  0 or 1, and its value is already reduced modulo the order of the field.
  */
 
 #ifdef VERIFY