docs: standardize description of flags

We had loads of different styles in describing the @flags parameter
for various APIs, as well as several APIs that didn't list which
enums provided the bit values valid for the flags.

The end result is one of two formats:
@flags: bitwise-OR of vir...Flags
@flags: extra flags; not used yet, so callers should always pass 0

* src/libvirt.c: Use common sentences for flags.  Also,
(virDomainGetBlockIoTune): Mention virTypedParameterFlags.
(virConnectOpenAuth): Mention virConnectFlags.
(virDomainMigrate, virDomainMigrate2, virDomainMigrateToURI)
(virDomainMigrateToURI2): Mention virDomainMigrateFlags.
(virDomainMemoryPeek): Mention virDomainMemoryFlags.
(virStoragePoolBuild): Mention virStoragePoolBuildFlags.
(virStoragePoolDelete): Mention virStoragePoolDeleteFlags.
(virStreamNew): Mention virStreamFlags.
(virDomainOpenGraphics): Mention virDomainOpenGraphicsFlags.