[re.regex.construct] Make constructors consistent.

Add "explicit" to the constructor descriptions for those
constructors marked explicit in the synopsis.

Fixes #48.
[atomics.types.generic]/7 Fix typo.

"a either" -> "either a".

Fixes #58.
[streambuf.virt.put] Fix typo.

"NULL non-value" -> "non-NULL value".

Fixes #63.
[atomics.types.generic]/4 Fix wording full->explicit specialization.

Fixes #77.